I'm the proud owner of this guy:
The description is as follows (emphasis mine):
1 bottom drawer, 1 shelf included SHIPPED LOOSE with no shelf clips,
the shelf is intended to be field installed to help hold the
microwave, this cabinet is intended for built in microwaves not
countertop microwaves, must field install microwave, this cabinet does
not have finished interior, cut out dimensions are 14 13/16" high by
17 3/4" wide, maximum cut out is 15 13/16" high by 20 7/8" wide,
premium UV finished plywood cabinet box construction, 1/2" plywood
hanging rails, 5/8" solid maple english dovetail drawer box with BLUM
motion integrated easy-close slides
​What kind of router and bit is warranted here in order to cleanly and straightly expand the hole?
Best Answer
Straightly has entirely to do with your fence/guide, and there are many ways to do that. You can fence the baseplate, use collars and fence those, or use bearing bits and fence those - it's just a matter of offset from the cut line.
Cleanly is trickier, and depends on where you need clean. Solid wood trim would allow you to cover your sins, while trying to get a perfect cut on both sides of plywood might lead you to a (usually comparatively expensive) double-spiral (upshear and downshear on the same bit) to cut both faces into the middle.
A upshear from the backside or a downshear from the face side is most typical, but may result in tear out on the backside. Unless your guests look at the inside of the cabinet which should be hidden by the microwave being in it seeking tearout, no-one will ever know but you, normally.
I do wonder if the opening is already framed in solid wood strips, looking at the illustration. That would make it much less prone to tearout.