I think that cost is not a big deal, those 220V plugs and 12 gauge
cords are not *that* expensive. What I want to find is an inline two
pole switch.

You can get a 2 pole toggle switch than looks like a fat light 3 way light
switch at any "big box" hardware store. They are rated 20 amps and should
cost less than $20. I put one in for my electric water heater to make it
easy to shut it off when I shut off the pump supply when I plan to be out of
the house for more than a day. If the rating on the switch isn't high
enough to make your happy, you can get a disconnect for a air-conditioner.
The "disconnect" is safer if you think you might have to repair the unit.

The box the disconnect is mounted in will provide the transition between the
flex and the romex.