I spoke with a representative from Yakima. They said the Replacement Spine and Rear Wheel Tray for Yakima HoldUp EVO Bike Carrier, part # 8880828, will fit your non EVO Yakima HoldUp 2. view full answer...
For your application I recommend swapping out the ball mount platform that came with the Stealth Hitches Hidden Trailer Hitch Receiver w/ Towing Kit # SH64GR for your 2018 Mercedes-Benz GLC300 for the Ball Mount # 391BMU3S which is 5" shorter than the one that you currently have and puts you within 1" of being level which is acceptable.
You can either swap out your current hitch ball or use the Curt # 40038 so you can change out your ball mounts for different trailers without reinstalling... view full answer...