Do they get knotted around you or will they stay put?
We have bought some double flat sheets to make into liners so that we can wash them half way through the holiday and freshen up the sleeping arangement a bit.
Me and my family have used liners, instead of sleeping bags when in Southern France during July, although these days me and Mrs Francais just use a large cotton sheet, far to hot in July in southern France for sleeping bags, although for UK , I use a Snugpak Merlin Softie along with a Snugpack silk liner, brrrrrr England.
I made some out of sheets, but only singles, which were just the right size for the sleeping bag. Like Angie, I can lay the bags out and put the liners inside to make sure they fit into the corners properly.
For the winter, I bought a couple of cheap fleece throws from Argos and made liners out of those and they're lovely and warm to get into, too.