I scratch built a stand-alone bunker for Legion late last year (pictured). I didn't really want to scratch build multiples so persevered as a first time Tinkercad user to get this designed. It's roughly 6" x 6" (I'll do the other 8" x 6" version when I have time). The roof just sits on top of the base (I glued mine down). Both files have been run through netfabb so they should print fine (they're printing ok for me).
If you do print it I'd love to see photos! :)
Done in a night experimenting with Tinkercad. The door and a couple of the greeblies are from 28mm Sci fi Greebles by jdteixeira.