You may want to reconsider your plan...
Ammo cans are freaking heavy and bits of your body do not fair well if it should become trapped between the can and say a rock.
I would take a look at some soft luggage options - there are a lot out there.
I use a Nomad Moab bag on my WRR (http://www.nomadrider.com/outbackbagmedium.aspx) and it is plenty. If I need more room I strap a 40L dry bag to the top and I am good to go. This setup has been on many rides with me - COBDR and TAT included and works well. More importantly it crashes well too.
The Nomad is a knock off of the Giant Loop Coyote bag which is what one of my friends has. That bag has more features and a higher quality than the Nomad but it does cost a few $$ more. There are a ton of soft luggage options out there - Nomad, Giant Loop, and Mosko just to name a few that I know work well.