I think that the shallowest practical plastic contain ers are the black-an d-clear clamshells that supermarkets sell "B achelor C hicken" in.
The black plastic tray on the bottom holds about 2" of soiless mix. The clear snap-on humidity dome slows down evaporation, so the shallow mix won't dry out AS fast as it would otherwise. But as soon as seedlings pop up, you should remove the rid to reduce humidity and damping-off.
I think that 2.5" is signifigantly better. The roots won't bottom out in just a few days, and you have a longer time window before you have to pot up. I prefer even deeper cells, but then I tend to leave plants in trays longer than is ideal, becuase I seldom have room to pot them all up and keep them under lights.
