All my stuff is for hammock camping so while I can't speak of sleeping bag "brands" I can speak materials.
DOWN - I only use it. Screw synthetics unless you're in a boat and will get your bag wet for sure or have no place to dry it. Just from breathing in cold weather the top will frost up and get wet when your breath condenses.
If you get DOWN go with 850 - 950 fill. Anything less is junk IMHO and when your life's on the line the 850 is the lowest I'd go.
If you have the ability to do so have them stuff an extra once in there. I get all my bags built with a little extra down.
I avoid all mummy bags. Way to hard to get in and out of. I have a 32* mummy but I hate it.
All of my bags have foot box's and no zippers. If you think you could get away with similar - here is who I use for my custom bags now. They offer 5" wider version for ground sleepers although I have not had issues with my normal width doing the same
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Here is who i used to use for custom bags but their fabric/down combination let's feathers fly so I changed brands. Nothing like waking up in black wool thermals covered by 100 white feathers
Lightweight Camping Hammocks | Backpacking Hammocks | Warbonnet Outdoors