Lots of different ways to make fish cribs. Remember that crappie like VERTICAL structure.
Before putting anything in the water you should first contact the DNR, even if it is private there might be laws regarding what can be submerged in the water. I'm sure as long as it is private, using non treated wood would be fine.
Best crappie crib I've seen was A big box made out of pallets. One on bottom, 4 walls, and a roof. Cinder blocks and half an xmas tree inside.