Motor control manufacturers such as Allen Bradley, Square-D, Cutler Hammer etc. Most have NEMA 4 (washdown / outdoor) versions of their pots for use with variable frequency drives etc. Here is one from Sq.-D, you would order the box separately or pre-built with the pot.
Vishay Sfernice make the P16 series of sealed cermet pots. The body is hermetically sealed, while the panel mounting is sealed by an O-ring. They have an integrated knob. Available from RS Components in the UK for £4.48 each, and no doubt from dozens of other places too. They are physically quite compact, and are of good quality.
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