I use a mouth diaphragm for a lope call. I will sometimes use a Primos call someone gave me a while back. It just sounds like a crow call any way. I used to have a hand made one that just had a rubber band. They all sound kinda like they are ok but really none of them sounds exactly like a lope challenge. However, the sound is not nearly as important as the proper language you use. The cadence and volume and number of notes is very important. You can get a call and instruction tape from Primos. I would guess you can find them in Cabelas or Bass Pro or a Sportsman's Warehouse or maybe even your local sporting goods store. In order to make a good challenge call you need to be the subordinate. If you make a dominant call to a dominant buck he will likely just circle and get your wind. The decoy only works about 1 out of 25 times or so with just the decoy. Then only about one in ten of the times its works will you get him to stand for a shot. Not good odds. The means for the most part there will be about a 1 in 250 chance of taking a lope with just a decoy. If you can learn the language of lopes and when to blow the challenge you can triple or quadruple your odds. You can also factor in the home range and the rutting core areas for lopes. It is very in depth what happens when lopes rut. Just because a dominant buck is dominant does not mean he will breed. Just because a subordinant buck is subordinant does not mean he will not have a harem and breed. Dominance has much more to do with age and body size than it does with horn size. Dominant bucks are generally 4 to 5 year old bucks and the biggest horns are usually on 3 year old bucks. If there is abundant nutrition and low stress then the older bucks will have the reserves after the rut to be able to go into the new year healthy and will then grow bigger. In low nutrition years (drought years) the 3 year old bucks did not rut the previous year so they go into the winter very fit and not run down from the rut. That gives them plenty of reserve to grow good horns for their 3 year old year. Their bodies still smaller than the 4 and 5 year olds. So with that fact in mind, the 3 year olds will mostly be bigger than the 4 and 5 in horn growth. By the time the rut comes in the older bucks have regained their strength and still have the herds of does -- provided they have a core area marked out that has does in it. The bucks will choose and mark out and zealously protect that area. Inside that area there is one dominant buck. More bucks smaller areas, fewer bucks bigger areas. Strangely enough the dominant buck will tolerate another buck in his area if there is a good reason for him to be there. The dominant buck will allow another dominant buck to come to water for a drink as long as he comes straight in, gets his drink and leaves without looking around. The dominant buck will blow a challenge when the intruder enters his territory. The dominant buck will not charge but also will not take his eyes of the intruder as long as the intruder just does his business and leaves. If the intruder returns the challenge call or if he deviates from the necessary travel route, the dominant buck will then charge. If the subordinant or intruder buck does not yield, then the fight is on. And boy do they fight. It's very vicious and death is common among two dominant bucks. Lopes do a lot of things the same as white tails while rutting. They do make scrapes at which time they will urinate and deficate in the scrape. They will either just before or just after rub the brush with the black patch glands on the side of their cheeks. The full blown scrapes are the boundaries. They are strong sign post markings to mark his territory. He will mark the brush every chance he gets inside his territory with his scent from his cheek patch. The full scrapes are for boundaries so are usually very visible. Often they will make these scrapes in the bare dirt in the two tracks of pasture roads. When you are hunting lopes and see the lope turds in the road, stop and check for urinating marks and hoof scratches in the dirt. This is a boundary marking. If you intrude on his territory with a decoy and blow a reply challenge when he challenges you, it can get really crazy. It is possible for the dominant buck to t-bone your decoy at 60 plus miles per hour. Rare but possible. By the way... the Mel Dutton decoy is the best I know of I did see one on TV a while back that might work too. Some sort of blow up thing like a beach ball or balloon or something like that. The soft pop up decoys do not work well because of the wind bending them all around. If you have no wind where you are then I would guess they would work quite well.
So with this knowledge, find the buck you want, watch him until you figure out his territory and if he is a dominant buck (dominant bucks are always marking and scenting -- always) in that area, close in to within 150 yards or so (undetected), when your buck is shielded and cannot see you, pop up the decoy and wait for the dominant buck to see your decoy and blow a challenge. When he blows his challenge then just duplicate what the dominant buck says to you. If you do this properly enough times it will put a dominant buck right in your lap. If you do get him to stand for a shot it will be quick so be ready fast. Also do not use expandable broadheads. Lope bones area really strong -- 23 times stronger than beef bones -- really they did a study. Use very good fixed blades and make a good shot. head on shots will be common that's why the good broadheads. A good well placed broadside shot will zip right through in the lungs. You miss a little or need to take a head on shot the mechanicals will give you trouble. It's very likely the mechanicals will stick in the near side shoulder blade if you hit it. I have seen it many times, thats why I do not allow mechanicals in camp. If you shoot mechanicals, you do not hunt with us.
Best of luck to you. Post the pictures. By the way - blinds on water holes in a dry climate works lots better. Decoying and calling is by far more fun and more exciting though.