here is a shot of a subframe i welded up to strengthen. the tabs for the canister/toolbox were re-enforced quite a bit. an otterbox was installed with stainless steel fabricated bracket to the two 6mm threaded tabs... over a few thousand km they fatigued due to the weight of the factory tool kit in the box and broke off without warning. the box jammed between the rear tire and inner fender, causing the back wheel to lock and stall the bike. damage caused to the silencer, exhaust, inner fender and tyre.
i would not recommend mounting stuff in there if it can be avoided as there is no safe place for things to exit... the only way out is between the tyre and fender!