Our 'boarding' cats have a carrier each, stacked on top of each other to save space.
One often sleeps in the top one.
The shells are plastic, quite sturdy, with plastic ring 'feet' on the top and bottom.
You'd think they would make them so they stack, large ring on the bottom, small on the top.
No... they are the same size!
So I printed some of these simple rings of the correct size to join the carriers together.
Now the top one doesn't move around when the cat jumps in.
CarrierConnector