Think the wider PVC is preferred because it adds stability and contacts more of the eggcrate distributing the weight better. If using smaller PVC tubes, would just add more of them in a similar ratio of the decrease in diameter. For example, 1.5 in. is half the diameter of 3 in.; so just double the amount of PVC supports.

Nothing needs to go in the false bottom space, it's there just to collect extra water until you drain it as needed (either through drilled hole and bulkhead or via a tube access and aquarium siphon from top).

Don't forget to ^ notch the bottoms of PVC "legs" so that water does not pool inside them. Another trick myself do is to very carefully (have someone with work gloves hold PVC and start drill very slowly or much better, use a vise to hold it) drill two opposite holes in the PVC top area, large enough to pass a very small and long enough cable tie that will hold together the PVC piece, egg crate, and plastic screen .