Note that I live in Texas - already having 100 degree days. They can be backed with anything, but I back mine with sheets. Sometimes use nicer fabric, but recently - sheets.
A summer quilt has a nice heft to it - not too flimsy - just right for a little cover on a hot night - even with our ever-present air conditioning. Good for naps, too! :) They also fold smaller than a quilt. I keep one under the seat in my van.
This one was made with 3 yards of fabric - it's a generous lap size. Had to piece the outer border on top and bottom so probably around 48X52 or so. Just right for a 4-year-old to curl up with.