HDPE and PP are the common plastics used for storage containers. These
materials are not easily or reliably glued. Most reliable joining is by plastic
hot air welding or similar techniques.
One old idea is filling up the space with ping pong balls or old tennis balls
for flotation. You might also find a source of industrial waste and look for
the blue plastic barrels that liquid materials are shipped in. These would not
need modification to float. Another way that works is to prepare a polystyrene
foam block of the shape you want and fiberglass it with an epoxy resin.
Polyester resins will dissolve the foam but epoxies don't. These materials are
usually found in boat shops. Another cheap source of containers might be
plastic gas tanks from a auto salvage yard. Sealing them up could be possible
using a heat source like a soldering iron.
Hope this helps...

Joe