Looks great but smells a little
This stool looks great and doesn't scream "cat" unlike many other dens and scratch posts (which I personally perfer). The kittens use it to climb onto the bed.
One negative I have found that it smells really weird. Maybe this is why they don't lie in it. I hope the smell will fade over time and they'll take to it as a nice little den. If all else fails, you can use it as a stool, it seems quite sturdy for a normal-sized person. The inside could also be padded, that would make it even cosier.