I had broken fasteners on some speaker grills some time back.
I drilled out the broken tabs from the speaker body and then used cylindrical magnets to go into the tab holes and stuck flat disc magnets onto the grills. Results were excellent. Also rendered the covers much easier to remove and fit.
You could also get in touch with Mr. Murthy who does a lot of audio stuff. He is on Commercial Street.
Search for his details on the forum. He has been mentioned several times.