Plastic speaker mounts, all the same?

Yes. All 4 doors have identical speakers and plastic mounts (for stock Prius - with JBL system rear speakers are different but have size holes for plastic mounts). Once you create the first wood mount you should be able to duplicate it for the other 3 speakers. However keep in mind the shape of the metal is slightly different on each side and between the front and back and your mounts will likely fit each just slightly differently. When I created my speaker mounts for my rear speakers, I used some liquid nails to fill in the gaps that occurred because the wood didn't fit quite flush. I have the door interiors almost completely sealed by using some 3/8" board and sound damping mat to fill all of the holes (there are still some very small gaps from the holes for the fittings for the interior door panel, and the drain hole at the bottom that shouldn't be covered). This creates a sealed enclosure for the speakers and increases the bass response.