For me females generally lay their eggs 10 days after their pre-egglaying shed (which is about a month after successfully breeding). Most people put in the nestbox after the snake has the pre-egglaying shed.

I like to use plastic shoeboxes with damp sphagnum moss. There is no ratio - the moss is dampened and then rung out so it is barely moist.

Some corns push all the nesting material away, like this amel motley of mine:

And this striped ghost:

And this Reverse Okeetee:

So I wouldn't worry too much about the egglaying medium, a small space with high humidity is what the snake is seeking.

I usually do not cut a hole in the shoebox lid, rather I offset the lid. This works fine for me:

If you're new to breeding snakes, I highly recommend that you get Kathy Love's or Don Soderberg's corn snake book (or both books).

Tim
Third Eye