I wouldn't recommend a cover - it will scratch the aluminum. I am using one of the cheap canopies consisting of metal legs and poles covered by a tarp attached with ball-bungee ties. Costco sells a standard 10' x 20' with 7' legs, which didn't fit my Airstream. Search the internet for "canopy" and you should find a local source for "custom" sizes. My 12' x 30' x 8' tall (plus the pitched roof) fits my 27' Overlander with room to spare. Cost was about $250.00. Your Globtrotter might fit in the standard Costco model if you could extend the legs...
Phil in Irvine
WBCCI # 9759
2002 34 Slide Out Lounge
2005 Yukon XL, 3/4 ton, 4WD, 8.1 engine