I am not sure of the dimensions but my bet is that it would work very well with the JG piece you mentioned. I saw it at Sierra (it is currently on display in their window with the new commandos). One thing that is difficult to tell by the pictures is the detail, you have to see it to truly appreciate it. It is one of the best pillboxes I have ever seen. The battle damage is great, but the spent shells on the roof as if somone was up there firing does it for me.
I hope someone gets back to you with the exact dimensions.