You can fill it in with thick Weld-on, or for better color match, dissolve a bit of the original plastic (cut into shaving so it will dissolve) in Weld-on solvent (like #3). Basically, you fill the gap, let it dry, fill some more until you build up enough thickness to fill the void. <The thicker you make the weld-on, the fewer layers needed.> Then sand & buff. This will fill the void, but won't be totally invisible.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.