The Home depot's here carry 4' x 8' sheets of styrofoam that are used for insulation. You can get asheet for around $10.

Go for it man. I did. Below is a picture of my range. It is 13 yards and I LOVE to go down and just shoot. No it does not help me with sighting in the bow but damn it is fun to just shoot. Also I was only able to pull 62 pounds last year. This year I can shoot my Trykon maxed out and it max's out at 74.6 pounds. I have is set DOWN to 67 for hunting this year. It is just a GREAT way to build strength and work on form.

I used a bunch of old VERY large boxes and stacked them up againist the wall first. Then just put the bag target on a box. I shot THOUSANDS of shots down there when weather is bad, hot or cold. On nice days I go to the club and work on distance. I have to say I am shooting better out to 40 yards then I ever have and I have been shooting for 25+ years. If you have any questions about anything with this let me know either by pm or posting here. I will be out in the woods some now so it might take a few days to get back to you. Good luck with your range.

Corvus, Don't feel bad. Your not the only one. I was just thinking of doing the same thing. Was able to only measure out 8 yds, but thought it was better then nothing. Like you I am switching hands. All was good till the wife asked what I was doing. "YOU ARE NOT SHOOTING A BOW IN THE HOUSE!" I'm still working on her. I was going to use those blocks that you can get from Cabelas and the likes.

Thanks for the tips, guys! I was thinking about those 4x8 sheets of styrofoam insulation from Home Depot when I was there the other day, but I wonder how many inches it would take to stop an arrow? I don't know how dense that insulation is. It seems like 4 or 5 of those alternated with some cut up cardboard boxes would do the trick. I could put an old left-over piece of carpet on the very back.

thats plenty good enough reason right there

theres a smatrass in every crowd

I wouldn't use the styrofoam boards as the main target but putting 2 or 3 of them behind a bag target or block as a backup for any misses would be better then smacking an arrow into the cement blocks.
I used a homemade styrofoam target once and it was really hard to pull my aluminum arrows out of it.

Right - that would strictly be a backstop behind my regular targets.

Corvus,
Try a search for Homemade Broadhead targets . I asked this awhile ago and my target came out great.
Basically , you sandwich foam( Boogie boards) , cardboard or whatever you can find between two 2 x 6's . One top and one bottom . . Compacting the material is 4 threaded bars , 2 on each side. Tighten the nuts , compress the material and shoot away.
I tryed cardboard but was really hard getting my arrows out[:@]So I found some boogie boards , softer foam , likeBlock foam,cut them up to fit and wha la.
Neat thing is you can make these as big as you like and there so cheap to make. The shipping on targets is more than the target itself is to get here to my door. Try a search, there some really nice photo's of the guys targets and idea's.

I agree...

It's better to shoot than not shooting.

Just put some big wood board behind the target. For target, are you shooting broadhead or fieldpoint? I usually stuff newspaper in a box and shoot. Been doing that for 5 years and didn't cost me anything. Course, afterward, you can put it in the fireplace and use the paper as fire starter.