I really liked madmac10's Mr. BulletFeeder controller mount for the XL750 (assume it would work fine with the XL650), and printed it for my XL750. I also have a RL1100, and the casefeeder support pole is square-ish, 25.99mm x 25.62mm, so I remixed this to fit the RL1100 casefeeder pole.

I've printed the original twice, and suggest adding a 18(x)-16(y)-18(z)mm box modifier, centered over the screw holes in each side, and set the infill for that to be 100% rectilinear. Make sure that the box modifier is completely within the original body halves, as I have seen strange things happen if it isn't.

.2mm layer height, 15% gyroid infill, PLA or PETG.

Assemble with a 10-24 x 1" socket cap screw (you can carefully use it to chase the threads that were present in the original model. It worked for the one I already printed, but I may add a hex-nut cutout on the half that isn't recessed for the socket cap.

I've included the .3mf file with the modifier box and infill, and a gcode print file. I'm currently playing with a .6 nozzle, so you'll have to mod your file and slice it for your printer.