I started investigating, and quickly figured out that I could easily make a small, one-turn tunnel with 2 pieces of plywood. However, that was going to leave a lot of wood left over; since we don't really have room to store it and I hated to just throw it away, I decided I needed to get it down to just one piece of wood.
Between Sweet Husband and I, it took a little math and head scratching to figure out the best way to do it, but here is basically how we cut the wood.
Although I'm very inexperienced as of yet, I think this tunnel will work, at least for my purposes right now. Moe--who is currently working on his senior earthdog title--isn't having problems with the additional tunnel length. If he was, I don't think I would use this short of a tunnel for fear he would balk at something longer when it came time for a test. Moe's major problem right now is recall, and I think we can work that in a short tunnel just as easily as a long one. Or, at least, I hope so.
There are a few more pictures here.