Some aftermarket speakers have a tweeter that sits above the rim of the speaker. This spacer provides you extra room so the rim of the speaker mounts flush to the under side of the dash of your Corvette C3. The spacer is 0.25" think. You can use your slicer to make it thinner or thicker depending on the dimensions of your new speaker. The OEM speaker has mesh over the speaker to keep debris out of the speaker. You can glue mesh to the spacer if you want.
This may also work with other 1970's GM vehicles that use 4x6 dash speakers. You can reduce the thickness to 1/16" (0.025') to make a test plate.
v3 - I increased the size of the screw holes.
v2 - I adjusted the opening to better match the size of a couple speakers I tested.
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