SEM 2 stage bumper repair epoxy. Any real auto body shop has it. its 2 tubes, with a special tip that automaticly mixes the two compounds together while you dispense it with its own special “calk gun” basiclly its like 2 calk guns with smaller dia plungers side by side that cradle the epoxy tubes as one unit.
scuff and clean the surface and blast this shit on it. its sandable too. very strong stuff. on the back side embed in some fiberglass matt for a backer and extra strength.
your talking a heat source and fusing the plastic and using a filler plastic also… different deal. but idealy stronger.