I recently blew the door speakers in my Australian 2003 Subaru Outback wagon only to find i needed to make a mount to allow the fitment of new aftermarket speakers.
The following STLs will fit the three-point fitment holes in BH Outbacks 1999-2003. You'll need to print two rear and one of each front as they are angled. The door skins will fit perfectly over the mounts no problem.
I printed mine in PolyMaxx at 75% infill as I wanted something solid to screw the speakers to, but that may have been overkill. No supports required.
I also suggest some sort of stick on foam strip like door sealer on the face that touches the door to prevent rattles and make a better seal for bass.