My infantry bases are 3X3 cm. and artillery pieces like ATs are 4X3 cm, so I needed a bunker of 4X4 cm. that could contain either, so I took a 40X40 mm. base and some plasticard I had lying around, and made this bunker.
Maybe is not historically accurate but it works!
Below you can see a Pak 40 entering inside.
Pak's view from outside.
HMG inside and officer standing next for scale.