Through a hydroponics dealer about $170.00. These I got from a drug bust.
also I overbuild everything for respect of earthquakes. All stands are screwed and have framing clips (Simpson ) to reinforce connections.
this bulkhead is oversized it is 1-1/2" the reason is that i need a good wide flange to hold the edpm liner in place. it will withstand the movement of working on the bell siphon without leaking. note the double gaskets.
because the threaded shaft was too short to make it through the plywood, edpm liner with the nut tighten flush i had to recess the area 7/16". it took two hole saws one larger then the holding nut and one just large enough for the threaded bulkhead. first the larger hole saw first to score the area then use a router to remove the material and recess the large hole, then with the pilot hole drilled by the guide drill bit on the hole saw arbor the smaller whole saw is used to drill the shaft hole.
notice the curve in the plywood, assemble the material so that any curve or warp opposes the forces that will be asserted against it by the future contents and how it will be supported.