I’ve been using blankets for a number of years both for climbing and kayaking, finding them much easier to use than a mummy sleeping bag. I guess this all started while using sleeping bags on walls in winter conditions, as getting inside with all your clothes on, water and pee bottle, gas canister, Inner boots, camera etc - while still tied in, can be a nightmare - plus there’s zero chance of zipping up the bag. In order to get over this, I just started to pull the sleeping bag over me like a blanket, leaving the bottom section partially zipped so I could stuff my feet in there. This allowed me to just get in and out of my bag the same as getting in and out of bed and was way less claustrophobic. The reason this system actually works is that heat rises, it does not fall, and so all heat given off by your body (remember a sleeping is not warm, it’s you that’s warm) rises up into the insulation. In a normal mummy bag, you will find that the bottom of the bag is warm, but this is just conduction from body contact, and that heat is simply transferred into your mat when not using a mummy bag. On the reverse side if you get too hot - not good as any sweat will need to be replaced - you just kick your feet or arms out. Personally, I find this design works brilliantly and have used both one-person and two-person blankets, including 10 days on the Troll Wall and 7 days on the Eiger in Winter.
One thing I like about the blanket idea is that allows you to be more active/reactive in the night - sort of on alert, something I don’t find with a mummy bag (which you generally don’t want to unzip). Being able to react is vital in many situations, such as getting up to dig in a tent in a storm, use a piss bottle or cover yourself while cooking.
Insulation wise I have always used synthetic fills, as I find their reliability in tough conditions a great comfort (a game-changer on long routes, where you may bail just because your bag is (or you feel it will) degrade due to moisture (inside or out). A synthetic bag - unlike down - is cheap and easy to sew on a hobby sewing machine from a kit (the Ray Jardine bag is excellent and great value - but go for the alpine weight if venturing high - Ray used his skiing to the South Pole and climbing mount Vinson). If making your own from Ray’s pattern, remember that this is a pattern for lightweight backpacking - not alpinism. I would add a strong clip loop to the bag as well as a large pocket to stow items (or a few pockets). I also have some velcro tabs that allow me to secure clothing on the inside to boost insulation (like a belay jacket). Also don’t make it too short, as you need to be able to sit up in it comfortably (in a sitting/alpine bivy you will probably have all your clothes on, which will deal with the odd draft). If you’re really snazzy on the sewing machine how about making the stuff sack integral to the bag.
The question I know you’re asking is will this be suitable for general camping and leisure use? Well the answer is yes I think so, as that’s the bag I use, and find sleeping directly on a mat fine, and wonder if the mummy bag is partly a form of psychological protection?
In September I’m going to start working with Montane on a number of projects, and we’ve talked about doing some very niche products, perhaps using Kickstarter (to prove they are viable and fund them), and this would be a great one. Watch this space!