How to rig a rain tarp without trees -- poles, maybe??

I had no idea prussiks could be used for tarps. How about Jumars?

I'm wondering why a tarp can't be flown using your TT roof as the high point, and then cheap extendable tent poles at the edges and corners.

Walmart sells a VERY high-quality 40X 20 tarp fairly cheap, and this will work fine for you. Question is where you will stow it when not flying it.

And ORbiker thinks he knows about rain growing up in western Oregon? HA!! We used to drive to Oregon to get OUT of the rain!