Savior19;1700706; said:
what you have now wont support ****
Spoken like a well educated engineer, no doubt.
What you have now will be fine for not flying apart, but it'll bow. I'd add braces like has been suggested, either screw 2x4's lengthwise along the top of both long sides, or else add a top piece that has cross braces. Don't waste your time bracing the corners, unless you just want to for your own creative benefit. You're absolutely right that you're better off doing any fiberglassing before you put any supports in the way... but I'd also like to say that the fiberglass is not a requirement, and it will complicate the build significantly. A few coats of epoxy, or gelcoat, or MFK Vetted your sealant of choice will be fine. I skimmed through this thread, so maybe I missed it, but how are you planning to work your window? 2 short ones, or one longer one? If doing 2 short ones, I'd suggest screwing a 2x4 in, standing upright between the windows, and then screwing your top piece to this 2x4 when it goes on... 2-pane fronts always seem like they're gonna blowout in the middle to me for some reason.... whatever you do, if you're gonna add any bracing inside the tank near or under the waterline, do it before you start applying your sealing.