Odd with the extra holes. I still have the radio delete plate but that only matches the large center square and two knob holes, which won't help you. Too late for welding up the extras. Yes, a thin sheet of shapable plastic and a dash color match coat would be the easiest. I just installed my classic radio with blue tooth and extras. I paid $350 for it 3 years ago and they want over $700 now.
Chip
'Rusto-Mod' '51 Chevy 3600 5 window | C4 Corvette front/rear suspension & drivetrain | everything else looks old and stock '92 GMC Sonoma GT #15 of 806 '91 GMC Sonoma GT Extended cab 1 of 1 Trucks, Trucks.....and more Trucks