We heard some kvetching out there about the corrugated cardboard sides that panel Everyday Studio's Cat Tree. Perhaps London's WowBow has a better idea with their cat scratcher, onomatopoeiacally named Skritch. Skritch uses sisal in place of cardboard.
WowBow's creed is that "design should not be ignored when providing lifestyle solutions for your four-legged family member," and they stuck to it with this elegantly modern cat scratcher.
Skritch is wall or floor mounted (depending on your cat's predilictions) and features a replaceable sisal tube with vertical ribbing that prevents claw snagging.
Unfortunately you may need to travel to London to get it. (WowBow has an online store but it appears to be out of commission at the moment.)