Taking your cue from Ansel Adam's then - he also used his sleeping bag when out in Yosemite for loading darkslides.
Where'd you get that darkbag @Woodsy? I have a tent one for my 8x10 film but it's a faff setting it up when I'm travelling, that king sized duvet looking thing would be perfect.
I got it from amazon - well, marketplace really - in the link below. It's plastic, just so you know. I opened it up when I got it and held just a single layer up to my eye and looked directly at the sun and couldnt see a think. I'm therefore confident the material is ok. I've not checked the corners or arm seals yet, but I have no reason to think they are not absolutely fine.
Amazon