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I always used 6" net pots and they were adequate (barely) for 5-gallon DWC. I also tried the net pot lid approach that provides a net pot in an integral lid, though these have some issues - especially when it comes to taller plants with large buddage that tends to pull the lid over to one side.
Get the 6" pots and attach them with bolts to your lids and you'll be fine...
and using a 6 in pot is about the right size in a 5gal bkt as the top of the warning label is 4 gal of water and that also turns out to be 1 in under the lid. It just so happens to work ot that way and a hell of a lot easier to do with an even amount of water for nuets....and try the dollar store i did find some really close net pots there all you would have to do is drill the bottom out good luck