I recently had a chance to try out my travel sock extenders on my 11 foot gathered end and they don't work, at least not when my underquilt is installed. When i get the zipper closed my underquilt is compresses so much that it doesn't retain any warmth. I had pretty much the same result when i tried my inflatable pad installed as well. When i tried it with out the underquilt or pad installed the zipper was still very tough to get closed. When i did get it closed i was right on top of the sock.

I'm using a 11 foot Simply Light with a 83% ridgeline.

If you plan on using this without any bottom insulation i guess it might work ok, but you can forget about any kind room for insulation