My local marina had this tank from an old boat. They gave it to me along with a water/fuel separator for nothing. Fits in the front pretty good. With no real good way to screw it down, we decided to foam it in. The bottom of the tank is open. I left it this way to let any water that may get in there still be able to drain into the bilge.I just have to trim it down and fill in some low spots.

The finished job.