Read the instructions!
I was a bit hesitant after reading some of the reviews about the plastic being so poorly made that it didn't fit together - however after reading the instructions, they state that this will be the case due to how the box is packaged, but once you fiddle about and align, it will line up and fit together.
My cats are ragdolls, the bigger one weighing about 6 kilos. She has plenty of room to move around in this, and doesn't hate it anywhere near as much as her previous softer case. Recommend!