Cade sort of has a point. A covered wagon top would be a snap to do. All U need are 3 or 4 wood or pvc "bows" and a tarp to spread over them & fasten either to the side of the car or underneath.
HOWEVER... at anything much over 5 mph, it will just tear up &/or blow off.
U can do it. U just have to put it up when U park jh& take it down jh& pack it when U head back out on the road.
I have seen a pickup with wood sides, bows h&a tarp over the wood part. But the tarp was laced down to the wood tight as a drum!