I've worked some more on my eurorack case. I've drilled holes for the PSU and bus board and someone gave me a hint to check out "tubed wood paste" (I have absolutely no idea what it's called in English) and I'm using that to fill out the tiny gaps between the frame and the box instead of hiding them with a very thin wooden panel which was my initial plan. It seems to work out pretty well but includes a lot of applying the weird wood-glue paste, let it dry, sand it down, repeat. I think it will turn out OK :)