I don't really know whether or not 42 inches wide is the sweet spot for your situation or not. As you can see from some of my pictures, if my knees are just right up to the hard edge of the War Bonnet bridge hammock, there is excessive coverage(with the JRB) well up above my knees in the edge of a hammock. If my legs are resting on the edge of the hammock, sticking out a bit behind the edge, then the quilt seems to provide just about the right amount of coverage. So it appears there might be a little bit of excess coverage, maybe, so you can get away with a quilt that is not quite so wide.

But the irony of that is that, if I had my knees up on the hard edge of a bridge hammock, I'm probably going to need a piece of closed cell foam – or something else – to cushion because otherwise it would be quite uncomfortable after a while. In which case, I'm already insulated. Whether by small CCF inserts cut out and placed under some pants, or by knee pads outside my pants, or just a plain old leg pad. Since this is a short version JRB Greylock 4 under quilt, unless it's not very cold, I would probably be using this UQ with a closed cell foam leg pad to start with. When I do that, if I switch to fetal position, I move the pad. Normally in fetal position, I only need the pad for my feet anyway, as everything else is covered by the quilt. But, I can easily shift the pad so that it is covering my entire lower leg from the knee down. Which will cushion that hard edge at the same time.

Is weight and bulk an issue here? And or expense? Because I can't even imagine the temperature rating of that (in my case) 0° rated under quilt underneath a pad that at least one user has taken to well below zero, all by itself. Using it laid directly inside of a bridge hammock so that he is laying directly on the pad. Though admittedly, some other very knowledgeable users can't get anywhere near that kind of warmth out of these winter rated Neoair pads in a bridge hammock. So opinions differ as usual. But regardless, it's going to be very warm, then adding an under quilt is really going to push it down there, possibly way more than you will ever need, unless you are a deep winter hanger.

So if it was me, I don't think I would ever be buying an under quilt trying to insulate that last inch or 2 on the side of one particular Hammock. And even though I already have one, I don't think I'd be hauling it along on any backpacking trips unless it was cold enough that I might also need it for boosting my pads temperature rating. Not when I can use some form of knee or leg pads to accomplish the task. And in fact, the pads would accomplish an additional task: cushioning the hard edge of the bridge hammock. But of course, that's just me. What are you thinking at this point?