use bondo made for plastic bumpers on cars, for extra grip, scuff plastic with 60-36grit grade, I use superglue/crazy glue on this then sprinkle plastic filings for extra grip, then apply your "bondo." to that surface, use 120 grit to finish off before primer
you should be able to the flexible bondo from an auto parts shop that sell parts, paints, oils, bit an pieces for cars,
good luck