Use the box side as a pillow.
Start from simple sleeping on your back…
or on your side.
If you are longhair, use the benefit of your coat and tail.
Remember that you and your box must nearly amalgamate.
Use your imagination: try S-like positions..
or C-like positions…
or even L-like ones..
If the box is rather small, try sticking your paws & tail out of it…
or like this (front paw)..
Invite your friend to join you…
Try disguising to avoid unwanted attention..
or you can hide in an absolutely unexpected box..
Even small boxes can do..
practice makes perfect. To get more experience, use as a box any structure you can find..