I may consider forgoing bivy if someone has Hexamid Solo TARP + bathtub floor.

This is for August JMT and it looks like mosquitoes are not going to torment me (I have to use a tent in that situation). So I'm looking for bivy/tarp combo but it has to weigh a whole lot less than my tent (21oz Duplex) to make it worthwhile.

Bivy: I'm small (5'8", 150 lbs) so standard sizes. I've never used a bivy before but I've frequently cowboy camped in Sierra. One thing I can't figure out from reading posts is whether I can simply sleep on top of the darned thing or if that is a bad idea. If not, I'm going to require a side zipper or some way to open it up a good bit. (Yeah, I could bring a ground sheet too but I don't want to.)

Tarp: My favorite would be Heximid Solo tarp since I love my Duplex but don't need the space or bug net. CF Deschutes maybe. I can't use solo-plus because my camera tripod (using in lieu of hiking pole) only goes up to 48".