The Cotronics Corporation seems to have provided a method for using pop-pop engines in boats with vacuum formed thermo plastic hulls.
Go to http://tinyurl.com/25vlllv
for information about their Plastic Bonding Adhesive and to http://tinyurl.com/27vjne4
to download a data sheet and instructions for casting in the form of pdf files.
Their castable ceramic foam is interesting because it would provide thermal insulation for a plastic hull and a bit of extra buoyancy for a capsized boat.