Unless your plexi is very thick I'd seriously advise against this. Having no kind of bracing directly behind it gives the potential for cracking with the stresses placed on the buttons, particularly with the narrow sections between buttons. Usually, when using plexi it finds strength ang rip by the fact the buttons themselves help clamp the plexi down to the wood backing whilst in use, and add resistance in all directions in terms of tensile strength.
If you really want to stick with metal, you would be best to weld a new plate over the hole, then blend it with 'liquid metal' type filler, not bondo. That way, when you re-drill the bit will bite evenly and create a cleaner hole.