After contemplating building one myself, I found this on Ebay and just spent the money. I figured the time savings more than offset the cost. The casters alone would have cost $20+. It also came with a cover. It works great. I don't have a high enough ceiling in my garage to have it hanging in the middle indefinitely.
Porsche Roadster Porsche Boxster Hardtop Rack Cart Free Cover Included | eBay