I don't know what the best way is...I can tell you the way I did it though.
I bought the
double plate (by itself) along with the
single mounts.
I got both simply because I didn't know how'd I'd end up mounting them and the way I purchased them with coupons, individually, etc. made them cost just a few bucks.
Once I got my Megawarrior, I made due with what I had. The double mount is pretty much required on top regardless of what else you do, if only to keep the two 8 shaped posts (the handle thingy parts that stick through the gas tanks) the correct space apart.
Underneath, I simply sandwiched one of the Megawarriors skinny cross bars with the round single plate (on each side) and the double plate.
I had to buy some 1 1/2" bolts to go straight through the plates and into the back of the 8 shaped Rotopax mount. 1" bolts might have worked but the 3/4" that came with the Rotopax most definitely would not. 2" bolts were too long and maxed out into in the Rotopax mount. Truthfully, a bunch of different lengths of bolt were required as I ended up bending the bottom plate into oblivion. Starting with 2" to bend and ending with 1" is probably the best bet - I don't know though. The bending *was* required since the Rotopax was easily shifted up to the point I tightened the bolts to where the bottom plates touched the top plate. I also used the plates with the bolt recesses facing down instead of up based on my screw length. A little more time and effort could have produced slightly cleaner results I'm sure. Two trips to Lowe's for $5 in parts was wearing on me so I did whatever worked the easiest and I'm completely content with the result.
I forget the thread pitch of the bolts, I wanna say 5/18 for some reason (is that even a size?) Anyway I took one of the short, 3/8" long Rotopax bolts to Lowe's and using their little thread matching thing found it easily. It think I still have the bag they came in...I'll update when I can.
Other considerations are how you have the basket mounted to your rack and/or where in the basket you want to mount the Rotopax. I need mine at the back which is right where I have the mounts to my Thule Xsporter. This meant that my Rotopax is tilting slightly forward due to the Yakima mounts making the basket floor an uneven surface to mount to. I'm not too worried about it, but it would have been nice to have the tank sit below the top of the Yakima fairing. As is, it's about 1/2" above it in the back (I have my fairing on the back of the basket.) Out of the directions it could be tilting, I'm fine with forward: it's a wing! Hahaha.
Rotopax also makes a thicker
round plate that may be of use to you. I can't remember if it was offered when I bought my setup but they seem better suited for clamping and are only $5 each.
Also note that I wasn't able to span two bars with the bottom plate. I needed it as far back as possible and that's where it lined up, a single bar down the center of the Rotopax. I'm pretty sure the round plate isn't long enough to do so anyway.
I suppose u-bolts could work but you'd have to get some very specific ones with all the parts and the crazy configuration going on. If I do anything different it will be putting some rubber between the plates and bar to keep it as tight as possible and to keep metal from rubbing. As of right now it hasn't moved, but I haven't beaten on it either.
