In the other option category, I use a 20X8 foot white tarp ordered from Home Depot's web site. The typical problem with tarps is that they don't allow ventilation and capture the heat inside. So using the 8 foot wide tarp just covers the top.
The cover has a pvc pipe spreader bar in the middle to keep it open on the sides and off the top of the camper. Shorter pieces of pvc pipe in the front and back keep it spread and provide attachments for the bungies that hold it down. The PVC is attached using fender washers and pop rivets through the tarp grommets.
A few things about the tarp vs a typical pop up cover is that I can access the door without having to remove the cover and it is less susceptible to tearing on the corners. It is certainly cheaper to replace if it does tear. The drawback to access is that the sides of the camper are not protected from UV. My storage spot is partially shaded, so the camper sides don't get direct sun.
I don't have any firm info on how much the white tarp actually blocks the UV, but it certainly does block some. If heat isn't as much of an issue where you live, then a dark tarp would block practically everything.
2012 Rockwood A122S
2008 Toyota Tundra Double Cab 4X4
Former owner of a 2002 Coleman Niagara GTE