Snugly

There are gaps between the boards in the box. If you fit them in snugly, you'll get more use out of the box.

The old woman was snugly sitting on a cushion.

That fits in there quite snugly.

Make sure your shoes fit snugly and comfortably.

The peg fit so snugly in the hole that it couldn't be pulled out by hand.

He’s wrapped up snugly in a cozy bubble of self-regard, talking for his own sake more than anyone else’s.