I can't find anything on the dimensions of the Arc side panel or any mention of the Arc being based on the Define chassis, but from everything I can see on the two chassis' I would say that the Arc is just a modified Define mini chassis much like their ATX counterparts. That's all based on just photo comparisons though so it's not in any way a 100 percent accurate science.
If you have a look at this photo of the Arc mini
http://img.hexus.net/v2/chassis/Fractal/ArcMiniR2/MiniR2-05b.jpg
and this photo of the Define mini
http://www.silentpcreview.com/files/images/fractal-definemini/10.jpg
Alot of the features of the two are very very similar, the Arc has it's SSD mounts drilled into the motherboard tray, a different top panel and presumably front panel of the chassis itself to support those plastic panels on the outside, but the mounts for the side panels and the similarities everywhere else on the case lead me to believe they are pretty much the same thing and the panels should be interchangable. But as mentioned before I can't be 100 percent on this.
Sorry I couldn't be of more help, I've been dying to get my hands on an Arc mini R2 forever now but they aren't really available in Aus yet so I've yet to spend hands on time with it so I can confirm these suspisions.