I've contacted GOG multiple times but they weren't interested so ¯\_(ツ)_/¯
KeeperRL on Steam is DRM-free, btw
Purchased the game yesterday (here on itch.io) and very happy with it!
I'm not sure (maybe I'm not familiar enough with controls yet) but I think there is no way to unmake storage squares.
Reading around on internet, some say "remove construction" will do but it doesn't. I've also tried "unclaim territory" to no result.
Could it be because the square is actually ouside of the mountain?