I am determined to keep this thing afloat. Front and back the Damper van has hollow bottomed extensions and while I could just cover the bases with plasticard and hope this is OK, not having complete confidence in my own workmanship, they have been filled with expanded polystyrene before covering.
It's a smelly job as the plastic solvent eats polystyrene. That smell is bad news too as I'm pretty certain it's highly poisonous. Work in a well ventilated area and don't do too much at a time I think is the advice (actually, don't do it at all is probably the correct advice but then who reads instructions). With a bit of luck this "belt & braces" attitude will do the job. If the plastic box leaks, the amount of water that can be taken on board is limited by the polystyrene lump contained in it.
This is actually quite a contentious move. Within the model boat community, there are many who will argue vociferously that you shouldn't use any buoyancy devices as the hull should be properly watertight. That's true, but if any problems occur this will mitigate against them a bit. Just don't use it as an excuse to skimp on the hull.