Imagine that my MV sits on the counter, about where the leather bag
is in this shot:

https://www.flickr.com/photos/lcb112...n/photostream/

I have never had an issue with a microwave walking or sliding from where
it was placed. Nor has the customer had a complaint of their microwave
moving in the 25 years that they have lived in this house.



I have a short piece 2 x 4 between the back of the MV and the
backsplash to keep the
MV near the front of the counter. Without it, the MV slides backwards
with use and ends
up too deep to be convenient. That extra 3 1/2" forward makes a big
difference.

Will backward movement be an issue with the cubby you built?


No. And there is no movement. The microwave opening is already there.
I am only dressing up up the cabinet opening so that it does not look
like a cabinet missing it's doors.