Which brings me back to the RotopaX. The 1-gallon (3.8L) module is by coincidence almost exactly the same size (9.5" x 13.5" x 3") as my homebrew rack. The next size up is 1.75 gallons (6.6L), at 14" x 15" x 3.5". While that would still fit on my rack, the overhang would interfere with attaching bungies etc., and for my needs the capacity is slightly overkill. An extra 5L is about ideal for the distances I typically reach from gas stations (hours of opening notwithstanding). So I opted for the 1-gallon can, and may consider making a larger rack sometime to accommodate the 1.75 gal can and other potential luggage refinements.
The standard mount is ordered separately and includes a variety of mounting hardware. However, none of it enabled bolting the mount from the top, which I wanted to do so it could be removed quickly and easily without reaching under (or detaching) the rack. Since the mount stands so high in the middle of the rack, it's really in the way of using the rack for a tailbag or anything else that doesn't have a large hole in the middle.
Of course, there were no off-the-shelf metric cap bolts available that were long enough to pass through the mount and into the rack. My solution was to countersink the bolt holes in the mount. This would be a piece of cake with an end mill--which I didn't have, or the milling machine to use it. The four-jaw chuck on my lathe came in handy and with some dodgy machining I was able to countersink some respectable flat-bottomed holes that fit the 50mm M6 cap screws I had.
The mounting holes in the rack then needed to be threaded for M6. Since the rack is only 6mm 6061 aluminum, I didn't think it would withstand a whole lot of abuse from the bolts whenever I attached and removed the RotopaX mount. It would be a real nuisance if the threads stripped out while riding. My solution here was to tap the holes with a Helicoil-style M6 insert which would at least provide some stainless steel wear surface for the mounting bolts (tightened with blue Loctite). The Helicoil inserts protrude slightly below the rack, but it's nothing serious. There are some press-in nuts that may work better in this application, but my local fastener supplier only had plain steel options which would corrode quickly. I'll have to pore through my catalogues for a better option.
Anyway, the can fits really well and ride performance is not noticeably affected. It's a pretty secure system. Unfortunately, now I can't mount my tail bag. To solve that problem, I'm playing around with a folding aluminum panel concept that can flip up over the RotopaX, has slots to mount the bags straps, and can be quickly removed without tools. It can be unlatched and swung out of the way to access the can, or removed altogether in seconds. By making it out of aluminum (probably 3-4mm 6061), it'll provide a solid base for the tail bag, additional protection for the can, and additional mounting points for bungies, etc. Eventually I'll have to revise my rack design to accommodate all these tweaks and eliminate the need for certain hardware. That'll be a winter project!