I know many sleeping bag questions have been asked, but I couldn't find much info on liners. We are planning on doing ABC in late March and I've read that it will probably be pretty cold at the higher elevations. I have a 20 degree F bag and I don't think that will be sufficient. Has anyone tried the liners that add 10-25 degrees F to a sleeping bag? I don't want to buy a colder bag that I won't use much, if ever again. I also don't want to rent a bag.