Okay so I have an 85 Sun Sport and I'd like to build a custom ballast tank using marine grade ply and fiberglass/resin and whatnot. Basically I want to construct a box to my specific dimensions to fit under and sorta behind my rear bench seat, then fiberglass in the box to make it airtight. I'm obviously going to have fittings for fill, empty, and overflow glassed into the box as well.

I'm very inexperienced at fiberglassing and stuff like this so I was wondering what advice you guys have. How do you suggest getting the box completely airtight (aka how will I get the last side of the box closed up)?

What materials do you recommend to do the fiberglassing/sealing? I've found some sites with the ply but I was just wondering if anyone has any suggestions for good deals they've found in the past?

Thanks for the help guys!