Plenty of inserts of different types out there, but thread pitch is going to be the big challenge here. The thread size/pitch of a standard 1/4 jack (which I'm assuming is the same size as the pot you're using) is 3/8'-32. Alphas and other metric types are M8x0.75.
There are a lot of insert types available to fit a 3/8 or M8 shaft, but just about all of them you'll commonly find are 3/8-16 or M8x1.25 pitches. That fine thread pitch in a 3/8 size is really hard to find in any style insert, much less one that will work well for this problem.
For that reason, I think finding something like what Gene shows here is going to be tough. Those are probably custom order parts from fastener manufacturers, if I had to guess.
You might find making your own easier. A zinc or aluminum pipe with a 5/16 ID and a 3/8-32 tap are definitely easier to get hold of (the drill bit size you'd use for a 3/8" tap is 5/16). You'd have to get creative with the flange part. Maybe cut across the end of the pipe in an "X" (maybe even bisect that once over with a "+") with a hacksaw or on a scroll saw with a metal cutting blade installed. Bend the cut ends out and flatten them, then drill a couple of holes in those flat flanges for riveting to the chassis. Cut the pipe to the right length and then tap out from the opposite end from the flanges. Not as sexy as a machined part, but it could work.
