gets started with WWI British in 15mm.
From Gallant Knight Enterprises
STL (3D Model) – $3.99 USD
This generic Small Bunker With Removable Roof is a 15mm (1:100 to 1:120 scale) STL model, suitable for WWI, WWII, Modern, and science-fiction games.
This model comes in two parts. Print the bottom part with the floor on the print bed, and the roof upside down.