Here's the speaker boxes I was talking about.
I find these "old school" surface mount speaker boxes from the 1970s from charity stores all the time when I find them.
But I couldn't find the correct size to go on the kick panels on my Trabant so I made some that would fit the car. I used some 3M adhesive tape on the backside instead of screwing them into the panels.
It's a multi-step process. I cut the boxes out to the size I wanted, tapped the outside to hold them in place, brushed resin on the inside, allowed this to cure, pulled the tape off and applied some polyester strips over the gaps on the inside and out and more resin. Then covered them with felt.