I have found one of the biggest reasons the detachable side plates may be hard to get off and on, is I that when you tighten the rear mounting bolts down, when you get to final tightness, the bolt end skews backwards.... which means the mount is no longer at a right angle to the side plate.

I attribute this to the fact that all the mounting hardware is flat 'washer type' of hardware, and none of it takes into account the slight inward slope of the fender strut as it extends backwards.

I'm in the process of making my own beveled washers to correct for this.

 

lemme know how it goes arch.
Mine fit like crap so due to trying to make them fit better I started a new mod for my bags.
Because i want to have quick detach for them too iam mounting them directly to my side mounts.
I finished cutting the bags and spacers just some drilling left.

 

Mine did the same thing. Angled back and made it difficult to mount the backrest. I put H-D saddlebags on and they straightened back to a right angle and it is now very easy to put on the backrest. You are on the right track with the washer idea.

 

humm... i dont have an issue with mine. did you try and install the side plates and then torque them down? that may keep them inline.