Jason, we built a box based on John's dimensions here. The only additional dimension that I see missing is that the rise from back to front is 4 inches. Ours ended up being constructed a little differently but is essentially the same dimensions. We used 1/2 inch plywood but found that the floor wasn't strong enough, so we put 2x4's across the bottom under the plywood every 18 inches or so. If you used 3/8 inch plywood, this probably wouldn't be necessary.
For the chains, make them plenty long, because they have to be able to adjust depending on the size of the lamb. With young lambs, they'll need to go all the way down to the top of the box. For older lambs they'll be shorter.
Also, we just bought this new "walking halter" made by Weaver. Used it once and like how it works.