We recently bought Ruby a new bed as her other one was as flat as a pancake. This one is relatively firm with a bit of squish and about 4" deep. The old but still usable covers didn't fit. Here it is laying on top of the bed:
Cut in half lengthways and a new strip added - now the new bed is inside this cover:
Took an age to work out how to join it back together so the bicycles were in the same direction. One more to do tomorrow using an old bit of red fabric then both will be done. The new bed has a furry, zippable, washable cover on it but I like to add another cover, just in case!