Hobby plastic sheet; cut to fit for a plug and modeling putty used like bondo on the gaps/imperfections. Putty can be found next to the modeling cements in the toy isles of dept. stores or a hobby shop.
Imperfections can be minimized some while adding strength by mixing super glue gel with plastic "sawdust", shavings, or baking soda. Much harder to sand smooth though ;putty is best for the final surfacing.
Very minor imperfections may fill in with paint or filler primer. Though the latter may wipe out detail, spoting/masking/sanding can cure that.