Here's a few quick pics of how i magnetized my drums for my troll. It took some time to realize how to do it simply (which is the way I like it) but it was only possible because I had a spare breastplate for my troll. I don't know if this will be helpful to anyone but maybe it will give some ideas!
It is easy to center the magnets because there's an even leftover "platform" from casting behind the breastplate which corresponds to the circular hole in the belt junction in the chest.
Without drums. The arms don't need magnets.
With drums. You can see the plate is a bit lopsided which is explained below.
Chop the small bar on the drum that's supposed to go into the holes in the troll's abdomen and glue the rest of the protruding part of the drum on the back/underside of the breastplate. Make sure to check the angles before you glue it in place because the troll's left thigh will make the angle of the left drum a bit complicated in addition to having to be able to fit the drum handle under the plate. It probably results in your plate sticking a bit from the troll's chest and somewhat lopsided but careful planning will make it minimal.