here's the rebuilt tab and screw hole.

Turns out tim Hortons ice cap cups are actually polypropylene, and as such, are compatible for use as a filler material.

Using a hot air gun, a soldering iron, and a putty knife I was able to melt down a large cup and knead it enough to become a solid, void free lump of plastic, about twice the size of what I needed. I also added in some steel wool to the molten plastic to kind of turn it into a composite, along with some of the black filler rod for colour, once it was fully kneaded I was able to get it hot and smush it down into a reasonably flat piece which I then cut and sanded to approximately the right shape, and then I welded it in using the same window screen, soldering iron and plastic filler rods method as I used for the cracks. It feels quite solid now, and I can knock it down with a file as needed if the latch for the case down quite go.