These walls have wholes to insert 3mm diameter magnets with a depth of 2mm. They seem common place on amazon. You should be able to make rectanglar walls of just about any size using these two files. Seems to look about right with a 30mm scale miniature game, but would probably work fine for 25mm to 32mm.

I printed my on a AnyCubicPhoton with a layer height of .02 with no supports straight to the build plate. Printing with anycubic's green resin this took 9 hours for a print.