Anyways... I seem to have quite a big zigsaw puzzle to figure out with that gunmount and recoil unit. None of the parts will fit together properly without some major modification. I think the easiest step, would be to just glue the original gun mount into the gun shield and cut it flush with the back of the gun shield. Fill the space around it and with the rear of the shield flat, I can just follow what Painless did to his Stug III. Why invent, what have already been invented, right
As far as I can see, the interior body is exactly the same, except for the cupola in the IV taking a bit more space than the flat hatches in the III, so doing what Painless did, shouldnt be too much of a problem...I hope
Have tried lining up the pieces, to get an idea of my options though... Have to make some weird cuts all around, if I really want the parts fitted together...
...have you thought about cutting into the backing of the mantlet where that wall is flat?...counter grind two dovetails to fit those gun holders...just the round part with the holes...or make new metal brackets that can be screwed on for easy removal...screw points could be fabricated and put on each side 
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