One way to fit something onto a smaller-diameter bar is to cut a strip of old inner tube rubber and wrap it around the bar. I have done this to fit headlights onto handebars, when the headlight clamp wouldn't tighten enough and the headlight tended to rotate down on the bar. I cut the rubber strip a bit wider than the headlight clamp, and just a bit shorter than the circumference of the handlebar. You can also cut it longer and wrap it around the bar a couple of times.
Matt, I have the regular Thorn accessory bars (items 11041 and 33509). I believe they both have a 22.2mm (7/8") diameter bar, which is a standard handlebar diameter. It is odd that the longer 45 degree bar that you have (item 28574) has a different diameter bar. Are you sure?
I have the Rixen & Kaul KlickFix mount for my Ortlieb handlebar bag, and it fits perfectly on the Thorn accessory bar that I have.
- Dave