Lots of glue...

Epoxy coated the watertight compartments and glued down the seat tops. Everything heavy in the garage used. 30 or so copies of my old dissertation, an old car battery, brake discs yet to be fitted to the car, some odd tools, etc... In the end I think I got nice glue joints although there was some mess made. 

The inside of the forward compartment. Lots of glue ouze. 

Oh, I used some clamps too