This is interesting to me as I just bought a Disney Stormtrooper Mask and have gutted the electronics from it. So, I am mounting the PCB, batteries, switch and speaker in a small ABS box, with the hope of locating it in my chest plate. If it doesn't fit, I shall have to fit it in the rear, like you - but somehow have the speaker at the front to hear it properly.
I trooped a third time this week and more of my trim dislodged itself. Make sure yours is well secured. I was given a good tip re the belt canister - stick some of the soft side of velcro on the clips where they contact the armour. This will stop any damage.
Have you started making your Plasma rifle yet?
Larry