The barrels shouldn't be too much of a problem. It should give you a chance to use some greenstuff. Figure out the size you need and work your way up. Start with the wood underneath, after that is set use thin plasticard that is bendy to put the metal band around the center.

Ben recently told me about rivets that come in little strips you can get from any hobby train shop. These little buggers will turn any piece of plain plasticard into metal. Little pygmies probably arent the most crafted blacksmiths so if your platform looks wonky it will just add to the effect. I would just glue sheets of plasticard together in the shape you need and add some rivets.

As for the magnetizing part get some high quality rare earth magnets. I use amazingmagnets.com . 2 of these magnets are powerful enough that they should work through a thin layer of greenstuff or a thick layer of paint. Drill/cut out the holes in the hands and on the top for the gunner. put the magnets in and use some greenstuff to fill the gaps. With no gaps and a decent amount of paint the hands will look fine. You dont need to worry about covering up the magnets on the barrels or the platform.