First off, that material is usually very light, snags and tears badly, and holds scent terribly, on the whole, the cheap camo stuff you buy at walmart is really bad for hunting clothes. Finding blind material made out of thick canvas/nylon for about the same price is pretty easy, and they make a lot better coveralls.

A really good idea is to take your work coveralls that you know fit well and measure and trace them so you get an idea of the pattern, then, add extra fabric to account for the seams and leave the sleeves and legs long so that you can cut them to fit later.

Remember to put in all of the pockets you're going to want in the places you want, and use quiet zippers, or double buttons with tight flaps, so that you can access your stuff quietly and still not have to worry about it falling out as you bend down, fall in the creek, crawl, etc...

One thing you might really want to think about: you're going to spend a premium getting really warm filler that isn't scent holding, so think about going to like a salvation army store or the like and buying old crappy looking ski coats, gut them and throw away the shells. It's also a good idea to cut the elastic wrist cuffs out of them and sew them into your coveralls, that way you keep your heat in, and you can overlap your gloves to stay warm.

My advise:

put thick patches of canvas or the like over the elbows and knees, maybe even consider putting some dense foam/rubber in there for padding, at least on the knees.

Also put a thick canvas layer around the cuff and up the leg a ways, I've even used leather around the cuff and canvas about 10" up the leg, helps prevent your cuffs from tattering before their time.

make a removeable hood, snap on works well enough, I make mine out of orange so that I don't have to worry about putting something over it to fit state laws.

Find a good seamstress to make it for you, it's pretty hard to get everything layed out right and to get it to come out right, I've been doing chaps and leather welding capes etc for a long time now, so I'm kind of used to it, it's a lot harder than measuring things out and stitching it together.

Put mobility pleats in the shoulders, maybe even down the back if you're on the portly side, that way you have a little more flexibility in them.

Put a heavily insulated " half-tube pockets" in the front of them, make it over sized with elastic around the openings to hold it shut when you're not in, and tight on your arms when you are, tube pockets are easier to get in and out, of while sitting especially, than hip pockets (like jeans front pockets). They don't hold gear worth a hoot, but they're great for being warm, and you should have pockets designed with all of your gear in mind other places (or do like I do and carry a pack for the bulk of my gear, and only have my little stuff on me.)

While you're thinking about making your own camo clothes, you might consider just buying a cheaper pair of canvas coveralls and some mesh netting, sew the netting around the coveralls like a shell, then buy a couple chunks of burlap blind material, shred it into strips and tie them into the netting, wash it a couple times and you've got a ghillie suit for under 100 bucks, you can also veg it out with pampas grass and small limbs by tying them in with the burlap, or threading them through the netting.