Use your router as a milling machine

A router can be used as a simple milling machine by constucting a jig to hold the router level as you roam over the job surface

The Jig

The jig needs to be wide enough top span the job when the router is moved to the far-left or far-right of the job
A strengthening rib glued to the top will prevent any sagging in the middle

Four screws are enough to hold the router in place

A single hole is all that is needed for the bit

Sitting on the sides of a box

In this scenario, the bottom of the speaker (box is upside down) had not been glued on square.
Up to half the thickness of the panel needs to be removed from one section
The sides have not yet been flush-trimmed, and are square, so can be used to support the jig.
To avoid cutting into the sides excessively, stops have been clamped to either end of the jig
By moving the jig side to side and front to back, the whole job surface can be skimmed

With the router screwed to the jig, vibration can be a problem. This pic shows where the collet holding the bit vibrated loose. Luckilly in this situation, a new panel is going to be glued over the old one, so it doesn't matter

Using guides clamped to the box

In most situations, you will need to clamp guides to the sides of your job.
These are best thought of as comprising the second half of the jig.

These port outlets needed trimming to be flush with the surface. Temporary blocks were used to get the height for the guides

A temporary flat panel sitting on the blocks allows the guides to be positioned against the bottom of the panel. This sets the guide tops parallel to the job surface

When the panel and blocks are removed, the jig will sit above the job by the height of the blocks

The result!
The ports have been trimmed to just above the surface without any damage to the panel
A little bit of hand sanding to finish, and the transition will be as smooth as silk...

This jig is much the same as the one detailed at Highland Woodworking to flatten a workbench with a router

For milling a vertical surface, provided it is not too thick, a bridge style support for the router does the trick

An example...
The protruding section needed to be cut off flush with the vertical edge of the triangular face

A bridge jig was made to sit over the job. A slot allows a long straight bit to sit through to jig and a guide rail is screwed to the top. The height of the jig was made to suit this particular job.

Jig is clamped over the job

Jig is lined up over the section to be milled

As Dubya said, Mission Accomplished!

Same jig, different job. Lot's of clamps...