Question about a DIY underquilt-type setup
The best advice is to practice at home. Start either in the back yard or an empty bedroom with the window open. It's nice to have the option of bailing when your bed is right there.
On my bike tour last fall I used pads so I'd have it to go to ground If necessary. On the last night of camping an unexpected cold front rolled through. I bought a cheap bag to use as a top quilt and rigged up my quilt as an under quilt.