to start the same project this weekend...
They recommended silicone sealant, no specific brand. I don't recall which I used, just a "good" brand from the hardware store. I agree with what LemmyK wrote above.
I drilled a 2" hole in the covering board, just because that's what I had at hand. 1-1/2" should work, maybe even 1-1/4". 1" is probably too small. With clean tanks and clear water, the best indication is in the reflection of a flashlight beam off of the surface of the water, so you need room for your light, and clear room to see the reflection.
There are some considerations in placement of the deck plate.
1) I wanted it to be close to the center, for best access, visually and manually, to all corners of the tank.
2) I wanted to avoid the molded-in model number of the tank, both so I could read it, and to avoid the possibilty of a leak due to the "texture" of that number.
3) Another surface defect occurs as a result of the manufacturing process. It is effectively the umbilicus, or belly-button of the tank. Besides a 1-1/2 or 2" circle on the tank top, there may be a cylinder of polyethylene 1/2" in diameter extending into the tank at that same location.
4) Finally, my forward tank is secured by a couple of planks, maybe 1x4, running fore and aft over the tank. I shifted one of those planks about 1/2" to allow the deck plate to be installed between them and not under one or the other.
The deck plates may weep a bit past the O-ring seal when the tanks are full. The clear cover will fog in some circumstances, making viewing more difficult. But with my 2" opening, I have no problem seeing the reflection of the flashlight beam.