I would like to mount a reflex sight in the rear sight dove tail . The problem is the dovetail slot is larger then other M&P pistols, approximately .693 wide . Having someone machine one is an expensive option . Suggestions ?

My suggestion is don't try it.
Any .22LR is a finely balanced blowback system.
Adding any weight to the slide will likely upset that balance.
If you must do it, one of the mounts that attach to the rail may be a better choice.

My $0.03, YMMV

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-jwk-
US Army '72-'95

The rail mount would be ok, but I would like to keep it holsterable.