This basic bivi bag is lightweight, low bulk and low cost
It has a tapered shape designed to fit round a sleeping bag. There’s enough room for a winter sleeping bag but no other gear. The sleeping mat goes underneath not inside this bag, which then moves with your sleeping bag.
The hood closes with a drawcord on the side, which means it doesn’t press into you if you end up with the bag upside down. The hood can be shut down to a tiny hole. Snugpak point out though that it shouldn’t be completely sealed as the fabric isn’t air permeable and so you could suffocate (something that applies to other bags tested).
In rain I’d turn the bag upside down so the entrance was underneath rather than close it up tightly. Breathability is similar to other basic bags.
The Bivvi costs just over twice as much as the similar Alpkit Hunka but weighs considerably less so if weight is important it could be worth the extra money. Packed up it’s smaller than the Hunka too.