Tips for a downfiring sub build that I wish I'd known the first time

Ran into almost the same thing with my downfiring 12. I had it sitting on thick carpet with maybe 35mm of clearance and the low end was mushy and uneven depending on where the box landed on the pile. Stuck a pair of scrap MDF risers under it to lift it about 70mm and it cleaned up right away, so the floor boundary was doing more than I gave it credit for. Worth playing with the height before you commit to final feet. On the finish I stayed with paint, but only after sanding back the first coat and thinning the second one. Two light coats rolled on with a small foam roller came out way better than my first brush attempt. The vinyl wrap looks tidy in your photos of the process though, might try that on the next box since my edges always end up a bit lumpy.

For feet, I put felt-bottomed rubber pads on mine from the hardware store, the kind sold for furniture legs. They grip hardwood fine and haven't left marks in a year or so. Hockey pucks are the cheap option people use around here too, though they can slide around on polished boards unless you add a bit of felt underneath.

Nice work on the wrap, that stuff can fight you if the box isn't dead smooth. I found a hair dryer helps a ton on the corners, warms the vinyl up just enough to stretch it round the edge instead of creasing. Also worth checking driver sag if you're using a long-throw 10, some of them don't love pointing straight down long term.

I was working on my own box last month and I used a lighter wood for the sides instead of the really thick stuff. I thougt that woud make cutting the pieces easier, but the thin wood made the whole thing feel floppy when I put the speaker in. It sounds like I needed to add some extra support inside to keep the cabinet rigid, because the sound was really muddy on my first try. If I use a smaller box later, should I still try to use thicker panels for the sides?

Update: tried the hair dryer trick on the last corner and it made a huge difference, the vinyl stretched round instead of creasing like the first attempt. The riser idea has me thinking though, mine sits on hardwood so no carpet pile to worry about, but now I'm wondering if a bit more height would still open things up. Might knock some risers together and see.