I used to be a sail maker for a rather large company. I was in charge of the roller furling jib division but we were located in the buidling where all of the spinnaker work was done so I know nylon a wee bit.
An underquilt has no load on it yet you guys are building it with 1.0 ounce nylon for some reason. We used to make light air spinnakers with .25 ounce nylon. Since it is not subjected to extreme wear, is not under load please tell me why you are building with what we used to use for heavy air spinnakers. 1.2 was for heavy air. 2.2 ounce was a storm spinnaker for higher that 35 knots of breeze.