I found a blister of Battlefront's 15mm German PaK36 37mm anti-tank guns in my stash of unbuilt stuff.
Regrettably, the crew figures supplied with it are mostly dressed in camo smocks and helmet covers, so they're not really suitable for my 1939-40 German army unless I field them as SS — which I am reluctant to do, because frankly the SS were a real pack of bastards. I don't really fancy the idea of commanding a bunch of war criminals, even in make-believe.
The figures are all glued to 12.5mm (½") steel washers, which fit into the 13mm holes in the bases. The MDF bases themselves are glued to thin card so that the figures don't just fall straight through.
The two holes where the gun itself will go are filled with a couple of plugs left over from the laser cutting; unfortunately I don't have very many of these, I didn't foresee their usefulness when I got the bases cut, and I suppose most of them were just thrown away as scrap.
On the base on the right, I've squished some figures down over some cling-film over the magnets to ensure that the epoxy isn't higher than the thickness of the magnets, but with the second base (on the left) I didn't find that a necessary step.
The magnets I sourced from China, very cheaply. It cost me about ten dollars for 500 magnets in this size.
I've left the loader figure in place during the gluing to ensure that the position of the guns and the loader figures is compatible.