Trapezoidal prism, nice fit I might add, in the space below the v-berth bed. Used 8-layer marine plywood, epoxied all sides, screwed it together--just to hold the shape, epoxied all edges inside, glassed bottom and all edges except for top.

See from above

From the top down, facing aft, you really see the trapezoid here; notice the fittings and hoses on the left (starboard).

See hoses and fittings

All of the hoses fit nicely on the starboard side of the tank, running behind the shower seat to the head, and up to the deck fitting for pumpout and up to air vent. Notice the red handle--thats a thru-hull for overboard discharge, while at sea. I put in a Y-valve, hard to make out, on the left to direct traffic to either the tank or over. I installed an electric head simlutaneously, so you can see some adapting had to take place, from 1-1.5 inch. I have pictures of the head as well, if anyone wants to see them.

See all finished

 Here the project is finished, put a piece of plywood over--not touching--the tank to make for a bed king-sized at the bottom to smaller than a twin at the top. Note, only think about this as an option if you want the bed like this permanently. It was for me--I have a custom fit piece of foam, so there are no creases or anything, and the space in which the tank went was wasted from the get go. Now, if seeing the tank bugs me, I can either scarf in a matching piece of wood, or put a piece of cloth or art or something to totally shut it out of the cabin. Finally can sleep in my bed again, and I have a working electric. Life is good.