Mermite canisters are also popular among the adventure touring set.
I looked at them and at ammo cans before I settled on mounting some old aluminum samsonite suitcases to my bike. I did this by using longer bolts (bicycle headset) and spacers through the standard grab rail mount points. I very much doubt that this style of attachment would work for something much heavier like an ammo can.
If you have access to someone who welds, it would be pretty simple to weld a couple of "L" brackets to the frame on each side. Make sure the outermost surfaces of the "L"s clear the shocks. Then you simply drill and bolt the cans to the L brackets.
I love the utility of the suitcases, BTW. I can carry a more or less full emergency/tool kit with me, rain gear, etc., plus bungie cords and a small dry bag if I need extra storage capacity. Then I have the completely empty one to stuff my jacket and gloves in, or to carry a bag of groceries. Since they're both lockable, I don't have to worry with anyone casually rummaging through them.
The cans would let you do all this, I'm sure. I think you're going to love the ammocans/hardbags when you are done.
MF Battery, Iridium Plugs, RR relocated, Voltmeter, 170/80/15 Kenda Kruz rear tire, DIY Samsonite hard bags, DIY shaved seat with Beaded seat pad ('cause that's how I roll) and the dreaded STATOR CHANGE.
Shining Black Bess '86 VN750, retired for parts after a fried stator and being knocked (kee-runch) in her parking spot.