NEMA 6-50 plug to attach to electrical conduit
OK, another swing at this given that you also want the NEMA 6-50 plug to be available to use with a portable charger if the main fails.
The cheaty way
I'll use a precedent here.  They make little subpanels meant to go at RV parking spots, which include all relevant sockets.  Different RVs require different plugs, so they supply NEMA 14-50, NEMA 14-30 and NEMA 5-20 at the same time. They are allowed to be fed by a single 50A supply breaker, even though if all 3 kinds of RVs plugged in, they could pull 100A.  Why is this allowed? Because only one RV will fit on an RV stand. If the 14-50 is in use, the 14-30 must not be.
So.  Hardwire it like I suggest, but instead of a blank cover plate on the panel, fit a NEMA 6-50 and hook it up so both the hardwire and 6-50 are live.  Label the 6-50 socket &quot;EVSE Only&quot;. It can't be used unless your hardwired charger is not in use.  Because it's only one parking space.
The non-cheaty way
Install exactly one of those RV subpanels that has at least 4 breaker spaces.  Change one of the panel front sockets to NEMA 6-50, and add an extra 50A breaker to feed the EVSE charger that you hardwire.  If it has a 5th breaker space, then support the NEMA 5-20 outlet also, giving  you another way to plug in.
If you really, really want the existing charger to plug in, then do hardwire it into the panel, but wire it to a flexible cord and have it exit the panel and loop right back into the panel  Noting that these panels have outdoor in-use covers, that can be locked, have the cord's starting point be somewhere protected.


Based on what you say in comments, your primary concern is armoring the installation against casual vandalism, e.g. cable cutting. Presumably our vandal is bright enough to de-energize a cord before cutting it, so he'd unplug a flexible cable before cutting, eh?
Use a permanent wiring method ... temporarily
In other words, overcome your reluctance to hard-wire, because hard-wiring is really the right way to do this.
I have the experience to hardwire a thing like this in about 2 minutes.  For me, it's not any harder than wiring a plug.  I do all sorts of temporary work, and it's perfectly routine for me to &quot;permanently&quot; wire something up, all spit-shined and inspection-ready... then, 4 hours later, &quot;permanently&quot; unwire it.
That's exactly what you should do. Bap in a knockout on the junction box, and wire it in that way; either cap off the receptacle's wires or replace the receptacle with a 50 cent blank cover plate.  If the box is mounted flush, then add an extension box and side-enter that, or get a blank plate with a knockout hole on its face. Depending on what works physically at the site.
And then, when you move, just unhook the wires and cap it off with a 20 cent knockout plug, in a neat and proper manner.  Done.  It's really no different than removing a chandelier.
It goes without saying that you turn off the supply circuit breaker before touching the circuit wiring. Also, NEC requires that EVSEs are dedicated circuits, and forbids multiple outlets on 50A circuits with a socket, so de-energizing this circuit won't affect other users.*
Why that thing is not available
The concept of a &quot;plug&quot; implies there is a cord attached to it which is rated as cordage, i.e. is made for frequent flexing.  Flexible conduit is not rated for frequent flexing; it is intended for fixed, permanent installation and its bendiness is only intended as a convenience to the installer.  As such, a plug for flexible conduit is a contradiction in terms.
At least in the consumer space.  ThreePhaseEel describes some industrial exotica, but it'll be industrial-priced, and still need a variance from the AHJ to use it.
You say &quot;It's mere coincidence that nobody makes it&quot; - no, it's not.  You may notice that every piece of electrical equipment (such as a plug) has a stamp that says UL, CSA, ETL or other marks (CE is not one).  These are Nationally Recognized Testing Labs (NRTL).  NEC 110.2 requires that every piece of equipment be &quot;approved&quot;. The instructions must be approved as part of the listing.  UL won't approve instructions that tell the user to violate NEC.  So the maker would need to find a use for this thing that does not violate NEC, get their UL listing based on those instructions, and then you would use the device contrary to its instructions, which violates NEC 110.3b.
So it isn't happenstance that these don't exist.  There's reason for them not to.
Or, hardwire to one box, and jumper to the other
However this won't work if your risk is vandalism.
Go to a real electrical supply house.  Get a NEMA 6-50P line cord with pre-molded plug, that uses proper cordage.  Also get the correct strain relief that fits that particular cordage ... and that's why I'm sending you to the electrical supply.
Now get a 4-11/16&quot; or 4&quot; deep junction box.  Fit the 6-50 cord and strain relief into one of its knockouts.  Then screw it to the wall within cord's reach of your NEMA 6-50 socket and within reach of that flexible conduit.
Put this EVSE's flexible conduit into another knockout.
Pigtail a #10 ground wire off the ground screw on the box,  then splice wires to match.
Voilà, now you can plug and unplug the EVSE at will, you are not flexing things not meant to flex, you are flexing things meant to flex, and it's Code legal.NEC 400.7(A)(6).

400.7 Uses Permitted.
(A) Uses. Flexible cords and cables shall be used only for the following:

(6) Connection of utilization equipment (that's any device which uses electricity for something) to facilitate (allow) frequent interchange.