@kjarn They are made of some sort of material that keeps the pieces from slipping too much. Some of them you roll up like a jelly roll and others are more rigid like a portfolio where the pieces go on one side and it closes and latches. I had one of the latter many years ago because I've never had a spare table to leave the puzzle out on. It worked pretty well. You can probably find them on Amazon to see what they look like.
@olivetreeann when the children lived at home and I needed my dining table I used a zip up portfolio type that I stored under the bed. Now that I’m living alone I have a wooden tilt frame that is permanently on the dining table. If I need the table it can be laid flat and put under the bed.
Ian