I ran into the same with the "D1 Mini". The few cases available were expensive for what they were and not a great fit for my needs.

If you have access to a 3D printer, there are some pretty good models on thingiverse and printables. But YMMV depending on which board you're using.

Another possiblity... I wanted to permanently put one in my car, so having some basic protection with the least amount of bulk was important. I ended up buying some shrink tube large enough to go around the entire board (IIRC it was 40mm, 2:1 shrink ratio), got everything wired up, slid the tubing over and shrunk it down. Could probably "double wrap" for more complete coverage. Again, this is board dependent and worked well for the D1 since it doesn't have pre-soldered pins.

Cheers!