Big thanks to dkperdue for showing the pix of his mod (here) I copied it and I'm quite happy with the way it came out.
I already had used this bag (Mt. Hdwr 0° synth.) as an UQ but didn't do anything but put hang tabs on it. That wasn't the best so I thought I'd mod it some. I've done the whole "cut the bottom out of a sleeping bag and put a drawstring channel in" thing and I didn't love the way that worked so I thought I'd give dkperdue's version a go.

I cut into the thing last night and went looking for my grosgrain and couldn't find it so I used some leather instead. I had to hand stitch the gussets in the corners because I couldn't fit it all under my machines foot. The rest I did with a machine. Its a bit tough to wrestle that old girl around, so it's not the prettiest, but I don't think it'll fall apart. I'm not too worried about the weight since this thing is so massive it makes its own gravity and has other, lesser underquilts orbiting around it. She's a car/canoe camper anyway.

Haven't done anything with the hood end yet. I can get a pretty good seal w/o cutting the hood off, so if I can avoid that I will.