They tend to be made of the same sort of thing as car seat belts, so they are strong and should be able to handle being moved from bed to bed easily enough. What's harder is the effect on your arms of doing the mattress lifting...

There are two basic designs: a one piece X shape, and a probably better two O pieces alternative. With the latter, the arms/ankles are always secured to the sides of the bed, whereas with the X you can (and can need to) secure them to the top and bottom of the bed.

Aliexpress can be a good source of this sort of thing, given they're almost always made in China.

The non-lifting alternatives are things that fit round the feet of the bed, but they rely on their being some form of foot, even if tiny.

I'm slightly surprised that I haven't seen any that fit around the corners of the mattress.

Are they easy to fit under mattresses I dont fancy fighting with every bed I use to try to put them in place and remove after.