I have started using a center line to hold up the center of my underquilt, and it has been working well for me. I use a HG incubator 20 degree and I noticed when I pull on the middle on the sides it definitely makes the bottom warmer and it seals much better. Also I can loosen the other suspensions because some of the weight is being help up by the middle line, and I think it makes it less restrictive when I stretch out.
I use the WB zip on underquilt, and usually I would have to leave about 20" unzipped to allow the shockcord to go over the ridgeline. I could feel the drafts when those zippers are open, mostly because the XLC is my go to winter hammock and I would feel it more during cold weather, of course.
So last weekend I used a soldering iron and poked holes for the shock cord to pass through and I could immediately tell the difference! When I tested it, there was no wind and it only got down to 39 degrees, yet I could tell it was much more sealed and less drafty. I already sewed in grosgrain UQ loops last year, so it was not like I destroyed a great hammock, but I know I added to it and I would just like to pass along the info. By the way, the silver you see on the holes is leftover solder from the iron, and it is firmly stuck in place. I like to think of the solder as bearings.

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