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I once reenforced a tab like that bay epoxying a washer on the inside.
I usually get some thin gauge aluminium and drill it like a swim cheese to use as re-enforcing, JB Weld that to the inside
I am pretty sure that cover is made from ABS. You could also use abs glue and reinforce it from the back, with a piece of screen or better yet stainless steel mesh to add strength. You want a strong but slightly flexible repair so it wont crack in the future. I think what you used may be too rigid for that application. A good quality 2 part epoxy can also be used with good results.Rick.