Well, I've posted this pic below on previous threads, but it's relevant here also.
Personally I would do like I've already done for my other hitch cargo - I'd make my own bicycle cover using a tarp, and cinch it with a bungee cord.
Consumer-grade sewing machines can handle an eight-dollar tarp from Harbor Freight. Tarp is thick enough so that it won't flap around, especially when held with a bungee. It becomes a simple matter of creating the rough shape and then sewing a few seams (instructions here
). Easier with straight-sided objects, but the same approach would work for a bike.