I dont know the effect of the acid on the fiberglass, and I'm not sure that the silicon ‎usually used for sealing the sides of aquariums would resist the acid, you may need to test ‎it first, I know it can stand for a big pressure but I'm not sure how it will react with acids.‎
I use fridge food keeping plastic boxes for my projects and for draining battery acids, ‎they work fine for that, they hold big weights and the tops can be easily removed and ‎most have their tops anti-leaking when closed. ‎
I never built a battery before so my input is only for the container