I was having some trouble with my Rumba not making good contact with the charging base but just putting it on a stand made it hard for the Rumba to get onto. It's not a totally perfect fit but it's close enough that allows the Rumba to get onto the pad and make good contact with it such that I don't have any problems anymore.

I personally printed this just a bit oversized, around 105%, so mine is a bit too big but 100% should work just fine. The rubber on the bottom of the Rumba charger keeps it solidly attached to the print so it doesn't move around even oversized like mine is.