That looks really good, nice work.
I just got a pillbox in my RC playground i am making, but mine is concrete haha. Unfortunately i will have to paint the cutout for the pillbox, as i can't break the concrete to make a hold.
We just had a plastic bucket and alot of left over concrete from another job. Left it a while then up turned bucket and round concrete pillbox. It is only cause my RC playground is staying outside all year round.
I have to say i think i will have a go at making a broken pillbox using your technique for some cool diorama

The door just looks superb.
Need to see some more pics of your guard tower. I too had the same issue, although i got my inspiration from some old PC WW2 games like COD & Medal of Honour etc