Honestly, if you have a beach towell that you can fold in half a few times, that'll give your knees enough of a break from the straight concrete.
As for supplies, yeah, a lot can be shared, the critical items are things like the scissors, stanley knife and roller (or something to press down with. eg: fingers/knuckles

)
There's sufficiently small a crowd tomorrow that we should be able to share around the bits we have to help get the day done in satisfactory time.
I'll bring a full set of stuff even though I'm not doing my car, so I can help with some bits here and there.
