Email them, tell them exactly what you want, custom.
They will make it to your specs. They are extremely good and extremely well priced.
Lewis Ciddor, winner of TD, for example, uses one for racing and loves it.
Yeah, I only knew about the Snowyside because I'd seen it on his kit list and I only knew about him after listening to Hidden Athlete podcast. I've emailed them about it.
They have an eVent bivvy but it's not in stock on their site. Under 400g though.
http://www.borahgear.com/eventbivy.html
Rab Alpine is
https://www.ultralightoutdoorgear.co.uk/equipment-c3/tents-shelters-c25/bivi-bags-c41/event-alpine-bivi-p1759