Black Water Tank Plan

[Wavery]

I'm going to start building the mold for a fiberglass black water tank.

Before I start building the mold, I thought I'd brainstorm with some of you guys.

The current plan is to build a single female mold 18" wide x 19.5" long x 5" deep.
My thought (at the moment) is to use one mold to build 2, 5" deep tanks with a 1" wide joining lip. The 2 halves will be glassed together after the top mounting plate is installed in the top half and a 3" F/G drain pipe in the lower half.

The 1" lip is designed to support the weight of the tank (mainly) on the upper aluminum floor plate as apposed to just dropping it down on the, weaker, bottom floor plate. A 1" wide x 2" high square aluminum tube will surround the hole in the floor and the tank lip will rest on that tubing. The tank lip and tubbing will be drilled through to accommodate 16, 3" long S/S machine screws. I'll use nutserts in the aluminum floors as apposed to just using sheet metal screws.

The finished tank will be 18" wide x 19" long x 10" tall. That should net me about 14G capacity.

I'm open to any/all comments. Please don't be shy about criticizing the plan. I'm looking for any positive, negative and corrective comments.