The insulation on the bottom of a sleeping bag does very little when you lay on top of it and compress it. It can fill some gaps but most of it is smooshed under you. It's further unnecessary because you already have an insulated sleeping pad to provide warmth under you. This fact brought about the camping quilt, which is a sleep system with no bottom. It has a closed footbox, but then the bottom opens up around the knee area, such the the sleeper lays directly on the pad and the sides come back together at the neck. The sides are kept snug around the sleeper with a strap that can go around the sleeper, or around the bottom of the pad and two snaps at the neck.
Camping quilts are a popular choice for backpackers, bikepackers and anyone else who is concerned about the amount of weight they are carrying. They have also become popular for side sleepers as they allow enough room to pull your knees up while laying on your side. Claustrophobic sleepers love them for the unrestricted feeling and warm sleepers love them for ease of venting. The split, vertical baffle system and a differential cut keeps down perfectly in place instead of falling to the sides or bunching up leaving cold spots.
Quilts can have some limitations for some people and that is where my false bottom bags helps improve upon this design. False bottom sleep system