We were installing a stereo in an older truck and ran into an issue where the radio was slightly deeper than the available space. Rather than searching for a different mounting adapter (if one even existed) I decided to design and print a spacer. The radio and opening were 1 DIN size and I made the spacer 3/8" in depth because the radio was about 1/4" too deep. It worked perfectly and we were able to complete the installation.
Please note that the setup on my TurboCAD output the .stl file in un-scaled units which the slicer interprets as mm rather than inches, so it was necessary to scale the object up 25.4x (X, Y and Z). It should be very easy to do in almost any slicer software. That would also be the time to add any compensation for material shrinkage as needed. If you need a different thickness, just change the Z scaling in your slicer.
**Update-** I have also included a version that has been pre-scaled by 25.4x that may work as-is as well as two versions that have a small lip on the edge of the front face to help hide the typical installation sleeve edge (if applicable). One has sharp corners, the other has some fillets to soften the edges lightly. So far it has held up perfectly in the sunlight and summer/winter weather.