How do you index a collapsible stock buffer tube with the MCX/MPX?

[ABNAK]

I have an MCX and ordered the AR buffer tube adaptor from Sig as I don't like the metal stock it comes with. While it will accept a mil-spec buffer tube, there is no way to index it straight up and down like on an AR carbine, i.e. it's not made to use the end plate that keeps a collapsible stock from turning.

Will the torque of a castle nut alone keep it in place? I was thinking about mounting it on the gun, putting some Loctite on the threads and adjusting it (by the old eyeball method) perfectly straight up and down-wise. Let the Loctite cure for 24hrs and then crank down the castle nut.

I could try to get all fancy and drill and tap a screw that holds it in place but that is a bit more of a pain than I'm up for.

I know there are a few aftermarket adaptors out there for AR stocks but they don't fold; this one from Sig will still fold. Just gotta find a reliable way to index it.

[ABNAK]

My gunsmith buddy ground down a washer with a taper on one side of it to fit inside of the adaptor. Took just a tad off the lip off the buffer tube (that usually holds down the buffer retaining pin on an AR) so it was a c-hair less prominent, but still there so it could "crush" around the taper on the washer. Put some permanent Loctite on it and cranked it into perfect up-and-down alignment using the M-1965 Eyeball Device and then tightened the castle nut.