Fitting 5.25 speakers to my Frontera but the mounting holes don't line up so I knocked this up for an adapter. Printed mine in PETG should be fine hidden down in the door temp wise.

Printed with 5 walls, top and bottom with 25% infill and it seems fine so far. Feel free to play around as you see fit.

There are two versions one with preset holes for my 5.25" woofers and one with no preset holes other than the standard factory fit