Problem with Panzers

So I've run into a problem with my plan to create transformable tanks. My suspicion that the German Pz III and Pz IV conversions were going to be tougher than the Allied tanks has proven to be correct.

The problem is that the guns come in pieces: a piece with the barrel (one for each type of gun) and a piece without (only one). I was hoping to drill out a hole in each piece and insert my magnets, but there just isn't quite enough depth. I tried replacing one of the magnets with a piece of steel, but it won't work.

I haven't given up on my plan though.

I'm going to attempt to cast the piece without the barrel. I'm going to create a mold and then use 5 minute epoxy to make enough parts so that each gun will have both parts. 

Wish me luck. My next posting with detail how I did it and how it turned out.