That's fairly simple. Almost any tarp you see in these forums will work for that. The best idea I have seen is to take off the front wheel and use the rear guyline over that for the back support, and use the frame with the forks on the ground to support the front guyline. Stake out the sides as normal.
There are many threads here discussing the merits of each brand and style of tarp. There are also many threads discussing making your own, which is a fairly simple process and inexpensive.
Most tarps are around 4 to 10 ounces, plus stakes and would fit in a seat bag quite easily.