I have a large (strong) grown up cat, who've had a major operation on his hind quarters, and needed a hard plastic cone to keep him from biting the stitches.
To keep him from pulling the cone off, I tied a long ribbon (actually a bandage) through the loops in the cone, once around his neck, crossing down on his chest - behind the front legs - and crossing once more on his back, before connecting to the loops in the cone again. Then it is neatly tied with a bow on his back.
It isn't super important exactly how you tie it - just keep it behind the front legs and cross onto the back. That way, when the cat tries to pull it off, it will pull against his front legs, and the cat will stop pulling.
This is a sketch of how the ribbon could run:
The red and blue are both ends of the same string - they start at the middle, just behind the head of the cat - and run across the breast and under the front legs. Then somehow they are tied together on the back of the cat. It doesn't matter exactly how they connect at the back, but I like to run them once more through one of the loops on the cone.
Technically it would still be possible to pull the legs out of the loops, and pull of the cone, but so far my cat hasn't figured that out.