- Regelbau 612 -
The Regelbau program was a project by Germany in the second world war. It was a series of standardized bunkers designed to be built in large numbers in the Siegfried line and the Atlantic Wall. There were multiple strength levels at which these bunkers could be built, with the 612 being level B (2.0 meter thick walls) and being primarily designed for the army(Heere). The 612 was a "Schartenstand für Land- und Sturmgeschutz", or a "Casemate for Ground- and Assaultguns"

- Schematic -
The schematic itself is made in 1.16.5, but I can also recommend using this in 1.7.10. This version sadly doesn't have "clean" stone stairs and slabs, but you can fix this with mods like Staircraft and Slabcraft. These mods also work really well with these schematics:
McHeli mod
McHeli German WW2 content pack

If you have any tips don't hesitate to tell me in the comments!