Whatever works is what you have to do sometimes. I made a removal base that could be clamped to my existing luggage rack for mounting a a GIVI top case. I didn't want the top case to be on the bike all the time...just for trips.
Anyway, I knew the case wouldn't open with the backrest in the way, so I put the GIVI mount on a T style aluminum mitre gage track so I could slide it back when I wanted to open it. I used a spring loaded knob fastened to a stainless 1/4 inch bolt where the bolt would project up through the base like a pin to locked the GIVI mount into place. If ya can't buy it, make it!