Card stock models of boats and ships provide one of the best sources of designs for pop-pop boats. They are mostly of the waterline type that could be supported by small rafts with provisions for propulsion such as battery powered electric motors or pop-pop engines.
A new home page picture shows a card stock model of an ACW ironclad ram and a unit pop-pop engine mounted on what is essentially a raft. Provisions for waterproofing and reinforcing the card stock model would be required, but there are waterproof synthetic varieties of paper and waterproofing materials that could be used on standard papers. Reinforcing could be done by cementing appropriate rigid materials or applying pastes that cure at room temperature. We have plans for several models of ACW ironclads and blockade runners in the files section that can be downloaded by anyone who would like to print and assemble them.
Best wishes, Frank