Fortunately I was able to do so without breaking anything and now the twin mount is still available for a potential future project :)
With the Mauser BK-27 in mind I drew a breakout sketch of the pieces and started measurements from the existing barrel.
From this I built the gun body that will hold the gatling barrels
Based on this I calculated the size of the side walls,
as well as the central section.
Then I mounted the outer walls - with 3mm spacers to give the walls a total width of 5mm.
Then I used .25mm plasticard to cover the gap.