Step 3: Create your mounting brackets
I Googled around and found where other people had made mounting brackets for hard bags or simply bolted them directly to the bike with no intermediate piece. What I discovered was there isn't much in the way of one size fits all for this sort of thing. There are "Universal" mounting brackets but they are only universal because they have moving pieces so that the bracket can be adjusted to fit any bike, but that doesn't mean they fit any box. Normally it looks like people will buy a saddle bag/box specifically for the hole pattern in their box and then purchase a universal mounting bracket that adjusts to fit. These brackets run anywhere from 100 bucks a pair for what amounts to three bars of steel with joints/bolts for adjustment to several hundred dollars for some more sophisticated technology.
We opted to take about 10 dollars worth of scrap 1/4" billet aluminum I had laying around my shop/garage and fabricate a couple of plates to act as spacers and to account for the fact that the side of my buddies motorcycle is not completely flat so there was no way the ammo can could just bolt to the side of it without being a total mess.
First take the sheet of aluminum and hold it up next the fender of the bike and mark it for length, which should extend at least 1" from both bolts so that the pieces is at least 2" longer than the distance between to the two bolts for mounting bags.
The width of the pieces is not super critical, at least a few inches though... the wider the piece the more will be pressed against the box and the more support the box will receive, but some will have to be taken away to account for the rails on these 40mm cans anyway. I just split the difference on the piece of scrap I had laying around and marked it up for cutting.
Once your piece is marked as shown in the photo, use a metal cutting wheel on your grinder to create the two rectangular pieces.
Once the two pieces are cut, set one aside for awhile. Take the other and put it up against the bike again. Using a marker trace out areas where you need to remove some metal in order to get as flat of a fit against your bike as possible given it's natural curvature or any protruding surfaces while maintaining a 1" diameter of metal around what will become the two bolt holes. Grind away that excess metal using a bench grinder or grinding wheel on your angle grinder.
Now, to transfer a good bolt pattern onto your sheets of metal there are probably a dozen options. We simply used a piece of blue painters tape. First remove the bolts from the back of the motorcycle for holding bags and the fender. Put a strip of tape on the bike covering both holes, and with the edge of the tape about as level to the ground as you can get it by eyeballing it. Now push in on the tape where the bolt holes are, strong enough to make a permanent inward dimple on the tape but not so hard you push a tear into it or mangle the tape in any way. Once that is done remove the tape from the box being careful not to let it curl up or tear and transfer it to the last piece you finished grinding. Then take a center punch and place it directly in the middle of the dimple you created in the tape by pushing in on the bike, and give it one good solid tap with a hammer on a solid surface. This small dent will help to start your drill bit precisely where you want it and mark where you want to drill... in the picture I marked the dents with a sharpie so they were visible in the picture. Now, making sure to set your drill press to the appropriate speed for drilling this metal and thickness, drill out where you punched using a drill bit that matches the diameter of the bolts. Once this is done make sure you can get the bolts through the metal and that they thread straight into the bike with no problem.
Now you have one piece more or less complete. Take the other rectangle you set aside previously and clamp it tightly to the completed piece (using metal clamps, this work gets hot and will melt plastic clamps). Using a bench or angle grinder work these pieces clamped together until they are identical, then drill out the same holes in the second piece using the first as a template. To drill the holes in the second piece, the most critical part, perfectly just keep the two pieces clamped together and drill through the existing holes in the first piece into the second piece. You should now have two identical pieces.
In order to prevent scratching of the motorcycle frame once the can is mounted we used a little adhesive (you don't need much, it will be bolted anyway) to put a very thin strip of a foam rubber material similar to neoprene but more like foam on the plates on the inside where the piece will go against the bike. The material we used came from a box that held a helmet camera I bought, anything similar should work. You could also just use a couple of rubber washers... or gasket rubber... this stuff was just pretty good since it would help form a uniform surface against the very slightly curved surface of the bikes frame. If it deteriorates over time I expect I would use some gasket material to replace it.
Now just use the drill to knock out the holes in the rubber material for the bolts.