i previously paid for my setup to be custom built only to realize how cheaply it could have been done. it was merely a coleman-type (not even the original coleman brand!) reservoir with the lids nicely cut for 6 holes for 3 inch net pots. the only other thing that required any effort was the small hole drilled to allow the tube from the air pump.
i know how to make this, only thing i'm still trying to figure out is what kind of saw do i need to cut nice round holes in plastic?
I went to the Home Depot and got a 3 inch round drill bit head. Its a big 3 inch circle/w teeth with a drill bit attached to it. Just be careful when your doing you initial drilling because the lid(s if you mess up and crack a lid) is going to slide around on you while the cutting is going on. The bit costs around 25-35 bucks some are cheaper than most, but if your not a handyman or a do-it-yourselfer, then just get the cheap one.
Once the holes are cut, a 3 Inch mesh pot should fit in the nicely. And if your pot has a problem staying in the cut hole you can poke holes in your mesh pot and run a piece of wood ( I used bamboo skewer) through it to act as rails and hold the cup in place.
Also, you might want to have a file or sand paper handy to file/sand off the plastic frayed ends for a nice, smooth finished feel.