Reply by Sawkerf

I always make drawers 1-1/16” less than the opening. The mounting tabs will flex inward a little to accomodate a slightly smaller box, but there ain’t no “give” in the other direction.

You should also measure both the front and back of the opening. If the carcass sides are a little out of parallel, a good fit in front might “pinch” the box at the back.

-- Adversity doesn't build character...................it reveals it.