I am considering making my own black water tank. Blackwater being toilet
waste.

I would like to place the tank under the sole between the centre stringers.
The lateral stringers are over 1' off center and 15" deep, giving me a tank
of just over 2'W X 15"H. Not sure how long yet but I want a finished size of
around 45 gallons. I would leave space under the tank to allow any water to
migrate to the back of the boat and be discharged. This would also mean the
tank would only touch the hull along the stringers.

My understanding is that I can make such a tank using fiberglass cloth and
epoxy. I would use marine plywood for the form and glass the entire inside
of the box and lid. How would I achieve a good seal on the lid? The plumbing
I think I can figure out: discharge, venting etc. Does any one have any
cautions, suggestions? I sure wouldn't want this thing cracking and leaking.
I should be able to place a toilet right over the tank to eliminate pumping
it from toilet to tank. Good idea?

Thanks