Yeah, I built my speaker enclosures out of 2 small pieces of ½" plywood for the sides and then boxed them in with a thin piece of abs for the speaker to mount to. I already had a pair of 5 ¼" speakers (not marine - will replace with marine grade when these quit on me. They have been laying around for years, might as well use them!). I used the screw bosses already marked in the speaker locations on the outside, but had to drill small holes to attach the plywood sides, if that makes sense. I stuffed the boxes with foam for better sound. I bought a CHEAP 800w (more like 100w) amp off eBay and wired the speakers to it. Mounted the amp on top of the glove box (had to drill 4 small holes). I drilled a hole in the box where the factory stereo mounts to run my mp3 player cable through. The amp came with a gain control, but I didn't install it because you pretty much have it turned all the way up all of the time anyway, or you can use your mp3 player volume control if needed.
2015 Ranger 570 mid