Bare bottom, so the eggcrate will be visible, or cut to be hidden by the rock on it?
Here's a photo of 2 inch ABS cut in half and approx 1/2 inch thick. As mentioned it floats so a dab of silicone or 4 hands are required to keep it in place under water until some weight is on it. I did not cut my pieces in half but just took a small section out to allow circulation. With such small pieces there's no room to add holes for circulation so cutting in half makes more sense. Doesn't have to be ABS pipe, could do the same with PVC so it would be similar in colour to the eggcrate.
