still working on an optimal rack system to hold this CTT up.
couple of things: the aluminium frame visible on the outside edge is 25mm.
Apparently there are bars in the frame all the way across where the brackets are, plus one in the center (found it with a studfinder...

) and of course at the periphery. Hard to tell what the width of the cross bars under the brackets is. Probably 25mm as well, maybe more. We should find out. Not obvious that the bracket is riveted in the center of the cross-bar, though. My studfinder suggests it's off a bit so guys if you want to drill in the center of those bars, better check first where they really are.
The plastic sheet on the bottom is not very hard or thick. If you have rack bars to put the CTT on, I would suggest that the rack bars be under a CTT frame crossbar, not on the side of it. I would also suggest you don't put all the weight on the brackets, which would happen if you use a rack bar across directly under the brackets. Since I don't know how wide the CTT frame crossbars are, this pretty much mandates the use of a rack bar fitting inside the brackets, but then I'd use a spacer on top so that the weight is spread over the width of the frame, not just supported by the brackets.
I've been playing with different types of rack bars that I have around, but so far it's not a great fit. Trying to use Unistrut bars but the dimensions are not good for this... The inside of the brackets is about 32mm or 1 1/4" but I'd like to put some padding in there, so use something a tad thinner. Maybe 1" wide.
As far as preventing side to side motion, I'll probably end up using a bolt across the brackets into the rack bar, but then one should be careful that the weight doesn't end up being all on the bolts...