Build plate holder, organizer, storage rack, whatever you want to call it. Easily holds up to 5 256x256 for the P and X series printers and is quite stable. Ive had mine sitting next to my X1 for a good while and haven't had stability issues with it wanting to tip over or anything.

You should be able to insert the plates in with either the cleaning tab facing up or down, but I've always placed mine in with the nozzle cleaning tab pointing down.

Print it out of whatever material you want with reasonable print settings, it really does not need to be super strong. 3 perimeter walls and 10-15% infill should suffice.

A brim shouldn't be needed for this part and I printed it without it since there isn't too much room on the plate for one but you can add one if you have adhesion issues.

I would also skip the filament calibration for this print so that the calibration lines do not interfere with the print.