Finally, we here at IGG have a proper playground for our Pocket-Tactics models! I've been designing modular terrain tiles since I joined Thingiverse, but I hadn't quite found the right formula/scale until now. We magnetized ours using magnetic business cards cut and glued to the bottom and that worked amazingly well. Those and the magnetic whiteboard set us back $60 uh-merican, which wasn't too rough on the ol' Ill Gotten Games petty cash jar (which, I assure you, is rather petty).
These are scaled for our Pocket-Tactics models (which is, maybe, 20mm scale?). If you want 28mm scale tiles, you might want to scale them up by 125% before printing. Oh, and all of these printed for us without supports.
SHAMEFUL PLUG: - Oh, and if you're interested in the RPG we use these for, you can find Wayfarer: Things Beyond Wonder here in our Etsy store, in both PDF and book form: https://www.etsy.com/shop/IllGottenGames
Also, these files are available for Tabletop Simulator, so check them out in the Steam Workshops if you own that game (and you should!)