Not just a "spare part", a piece on itself!
The Box itself is covered with sisal carpet. Which is very strong. On the inside you'll find a very large area for your cat(s) to relax and chill. With the dimensions of 50 x 60 x 91cm, easy to place in the house or in the cats own room. The oval shaped openings in the front are 25x20cm. Which is big enough for the larger species! Your cats will be able to play around this reasonably compact box with ease!
Because this part is also a spare part for the RHRQuality Cat Tree Cat Paradise, there are pre-drilled holes for connecting this to the original cat tree. But maybe, you can create something with this as well...