MRHELLO,
I don't have any blueprints or anything, but you can look at pictures of them online. They're just wire. They resemble a minnow trap, but they are built a little differently.
I think that by the time you got done buying the wire, cutting it, bending it, fastening it, and poking holes in all your fingers etc that you would be better off buying one.
If you want to go really old school you can just throw a couple dead fish in a burlap bag, tie it off to something so that it doesn't get dragged off by anything, and then put it in shallow crayfish occupied waters with a few rocks on it to hold it on the bottom.
The crayfish will go inside to eat the fish and all you have to do is come by and pick the bag up and you have them all.
