Well you can have a look at Great Models:
Use the Database (search) function and search for stuff like:
or just experiment searching for whatever you like
and you should get a few results from Verlinden,
Custom Dioramics, etc...
One I noticed was from Custom Dioramics Kit# 7011,
Longues Battery Atlantic Wall Bunker - $95.95
There is no picture so I don't know what the heck they
are charging $100.00 for. Kinda expensive whatever it is.
Also, if you are into scratch-building, there is an Airfix Kit#6706 - Coastal Defense Fort. They have a picture of the box. I saw a review somewhere on this and the fort (coastal emplacement) looked really good. Had a removable roof also. It is in 1/72nd scale though. So you would have to measure all the pieces and then scratchbuild duplicate pieces double (approximately) the size.
Anyway, have a look.
BTW, I am not suggesting that you have to buy from there. I have ordered before and was pleased with the service. I am just referring to that site so you can search around and see if there is anything you can use.
Once you have the Manufacturer's Name and Product Code you can get it from there, or any other place you want to.