My thought trail was this will give me both a very high strength bond, as well as a bit more structural reinforcement. A bit more added weight sure, but it's not going to be super heavy anyway. However, I'm a little unsure as to how I would go about using the foam. I've played with the stuff before, and it has the potential to make quite the mess if I don't get the two halves together in time. Would I be better off leaving the parts untrimmed before bonding, then trim them after? To limit mess and make sure the foam doesn't spill out of the part and lock them into the mould? Or is there a better way to prevent foam leaking out?

I've used that estimation for bouyancy reservoirs on boats. As density is normally not accurate, it´s advisable (sorry if not correct) to leave exits for foam excess. If not, expansion pressure may damage the joint.