Well, traditionally they just used blankets to cover the bender, and since most blankets were of felted woven wool at the time, usually with a good bit of the sheep's natural oils left in, they were fairly well water resistant. However... since I don't have access to that, and will be substituting more modern (camouflaged) frame material than willow branches, I should be able to sandwich a layer of tarps between the outer covering (woven blankets) and the inner draping (vintage saris). I will be closing the ends at night with tarps, most likely, unless it's very warm, in which case I'll just use more saris to let the breeze through.
I've designed a frame that's theoretically easy to put together and take apart, and small enough to travel with, as well... but so far I haven't had the time or the spare cash to get everything I need for it. It shouldn't cost much, but there's always something more immediate that needs paying for, so we'll see if it happens this year.
Potlucks, on the other hand, are always fun and easy to work into the budget! We will be delighted to attend!