I would try and make it round. Why? Dr. Laura Tiu once told me they put square containers in a tank for bluegills to spawn on and they "turned their noses up" at them. As soon as they put round ones it they began spawning.

If I was doing this I would cut of the bottom of some plastic drums and fill them with gravel.

Bill Cody is a good one to ask about this.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.