Connecting the four terminals of a 14-50P to a homebrew 240v system with three wires
If it's 240 volt it doesn't have a neutral. So your home brew has 2 hots and a ground.

Just wire it with the reverse pattern of the receptacle except for the neutral (white) wire will be deleted from the plug.

10 gauge is good for 30 amps but understand the receptacle is protected with a 50 amp breaker so if your homebrew has a recommended max size breaker less than that you will be exceeding the recommended protection for your brewer.

Good luck!