Dustin,
I think you can accomplish your goal with little trouble and money. I too am working on a non-existent budget! I assume you're going to be skinning the walls on both sides with a finish grade ply? If so, I would examine whether your design needs a 5' height. I originally thought so, but redesigned my floor so I could accomplish both good sitting headroom and a small skirt to cover my frame. I was able to do so in just 4'.
5x10 or even 4x10 ply is getting harder and harder to find. My wife works for a lumber company and all of their suppliers have stopped producing oversized ply. I have chosen to join a very small piece of ply to the end of a 4x8 sheet to get my length. It only ended up needing to be 18x31" due to the angle of the rear profile. I am biscuit joining the two pieces. If you don't have a biscuit joiner, you can get a cheap one at Harbor Freight, or use a table saw to cut a slot in the butt edges and insert a thin spline along with some Titebond III. It doesn't need a ton of strength in that direction as your main forces will be vertical. If you are skinning it with finish grade ply, you'll be just fine.
By the way, I think if you have enough workspace to house a tear, you have enough to do any of this...I work in the space of about a one-car garage. The shop is bigger, but it's filled with stuff!
While Jim's solution of waterjet cutting is quite cool, you can easily cut this out with a jigsaw. Hopefully I'll post some pics on my thread of my walls cut out by Saturday or so. I have no time to play before then.
http://www.tnttt.com/viewtopic.php?t=27225
Check out this thread that give some good ideas:
http://www.tnttt.com/viewtopic.php?t=27570
Hope that helps!
Brad