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.
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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