I have tried to do the same, only putting the keystones on this model:
https://www.printables.com/model/993377-1u-rackmount-for-hp-elitedesk-g3-mini-and-newer-le
it seems like I'm unable to wrap my head around how CAD programs work, since I can't get it to work at all. Could anyone help me out with the Remix, or point me to some CAD tutoreials which doesn't focus on drawing objects from scratch.