The diffuser grid is used for substrateless method where you can use water only or very wet substrate as long as it doesn't touch the eggs.

The substrate method would be vermiculite mixed with water on a 1:1 ratio by weight. Bury the eggs half way in a tub of 2" mixture. You can also use hatchrite instead of manually mixing. You may also use the diffuser grid with this method.
Place the mixture or unopened hatchrite in the incubator a few weeks before egg laying so you can monitor and calibrate your temps.

Humidity will hold using both methods, no need to add holes in the egg tubs, you open the tubs every 3 days to check on the eggs and for air exchange.