tail light fillers ina step side

got an 88 chevy fullsize with the fiberglass step side bed. i want to put caddys in it and am wondering how to bond the fillers to the fiberglass bed sides? the top and inside towards the gate can be welded in but the outside and bottom will have to be bonded to the fiberglass. should i use fusor and if so should i tack the fillers then cut for the buckets and fully weld them before i bond the fillers to the fiberglass or is the fusor not affected by heat? thanks for any help.

lots of people have done this can someone please help me with the best way to shave the tail lights???