@Deny Here's what I have for hard top pics. I think it might have been better to make the cut further forward, maybe just an inch or so back of where it starts narrowing above the doors. Tops have kindof a weird curve to them, so when you cut out a chunk or add a chunk the curve doesn't alway come out fair. Further forward the curve might be more consistant. This isn't too bad tho, a smear of 410 and I think it'll be fine for the birds that'll be s***ting on it.
Inside, after clamping it straight I tabbed it with some strips of cloth to hold it so I didn't have to leave in screws or gaps around clamps. Started with a strip about 2" wide, then went out to about 8", with some manky old cloth that a rat had pissed on.
Then just sanded off the pokey edges and flipped it over. Didn't get much for pics of the top, but basically, just sanded it and tried to make a bit of a v, filled gap with epoxy and 410, then a strip of cloth.
I cheated and did a bit of a flood coat of filler when the epoxy was starting to kick. Just means I don't have to wait and mix more goop tomorrow, other than to fill whatever needs it to be smooth again. Hopefully after it's sanded fair there'll still be a bit of that cloth on top, but I don't think it's all that necessary for strength. Even with this minimal glassing, this is probably the stiffest section of the lid. I hate sanding so I didn't sand the gelcoat all the way back everywhere, theoretically you should, but I figure most of the overlap onto the gel will probably get sanded off anyways when fairing. I've seen other guys not even bother with cloth on top at all, just filler .
This set up with the pumps is the cat's ass if you use epoxy a lot. Ratio isn't very flexible with this stuff, so this makes it idiot proof.
I haven't been using it as much lately because I find titebond is just as good on wood and easier to use. If I didn't have this kicking around I'd probably have used polyester resin. It's cheaper and pretty much the same deal. Cut strips of cloth ( you can buy it various widths, or just get a couple yards off the roll and cut with scissors. The narrow width rolls are cleaner and don't fray, but the bound edge leaves a ridge, so it's a trade off. I generally prefer to cut strips myself rather than use the tapes ) and wet out with as little resin as possible so the cloth doesn't end up floating up from what you're glassing...and try to stay clean. (good luck with that

)
Hopefully that helps a little. Biggest trick is getting it to go together as fairly as possible before you even think about glue. You can use strips of wood and screws to temporarily hold it straight, I just used some of those little buttwelding clamps that harbour freight sells.
Lastly, end of tape is pretty much even with back of tub. Hard to hold and take a pic...
It's possible I planned it to be 18", but I doubt it.
My vent windows are narrowed by 1.5", if you want full sized ones I think you'd need that measurement to be 20" (or more)