Mine is a simple cube made of wood and filled with rubber mulch. 30" cubed. Haven't had a chance to test it yet, but this is a pretty common design. If need be a steel plate (probably no thicker than 1/4") can be placed in the back for those higher velocity rounds that make it all the way thru the mulch. Like I said, I have not tested mine yet, but plan to when it warms up. I will be sure to give a full report after testing.
(I wanted to make mine portable and relatively easy to load and unload out of a truck. If you are looking for a trap that will be permanant, I would go with a more elaborate and durable design).