Change a outside 220v line from hard wire to a irrigation motor to add plug and receptacle
First -- you just need a solid place to attach the box to.  A 4x4 post cemented into the ground in the correct place certainly qualifies! (It also means you won't have one of those disasters where someone nails a conduit-supported outdoor receptacle with a piece of lawn equipment and the conduit breaks, leaving exposed wiring around)

Second, you'll want a weatherproof box (they all have conduit knockouts) and a "while in use" bubble cover as this is a wet location.  (You may need a reducer, though, if your conduit is too big for the weatherproof box to fit onto it.)  Use the same material for the box as the existing conduit -- metal boxes for metal conduit, PVC boxes for plastic conduit.  The box should have holes on it which can be screwed to the 4x4 -- if the hole spacing is too wide for the post, simply use a piece of scrap 2x6 as a mounting-bracket.

Third, you'll be putting a single 6-30R receptacle in this weatherproof box, and using a matching 6-30P plug on the motor's cord.  (NEMA 6 is a 3-wire HOT/HOT/GROUND receptacle -- this is correct for a 240VAC only circuit.)  You may wish to use a L6-30 instead of a straight blade 6-30 here, as well, considering how infrequently this device is being plugged and unplugged.

Finally, make sure there's a small hole in the bottom of the box so that water that does enter can leave promptly (no bigger than a quarter inch).