Silvan wrote:
Has anyone done something like this?

Yes I have but a variant that sits/sat on saw horses. These
are kinda cool onna 'count of they only take a couple of
hours and you have something when your done.

Where are the gotchas?

What you are contemplating is a Go To Hell Router Table, or,
The Anti-Ultimate Router Table. These come in many flavors.

The Ultimate (lowest ranking that is) Go To Hell Router
Table is a router clamped in a bench vise. Some people
don't like these onna 'count of its not too hard to deform
the router casing. A step up from this would be to build a
small cradle to nest the motor in thus distributing the
pressures.

Next up comes the Slab Style Go To Hell Router Table. This
is nothing more than a 3/4" sink cut-out from a kitchen
counter with a router screwed to the underside and the bit
poking through the top. Use drywall screws to attach for
that Real Go To Hell Router Table Look. In use you simply
clamp this to a bench. Some people will warn you against
the motor being too heavy and bending the top. This is true
but for most cuts not a deal breaker. Also note, you will
need a little extra material out the back side to clear the
clamps and your fence.

After this comes the Re-Enforced Slab Style Go To Hell
Router Table where you re-enforce the slab all around with
some lumber set on edge and glued and screwed to take the
flex out. Really anal Go To Hell Router Table makers will
even go so far as to build a torsion box for this. That's
too much work and the end up with something that's not Go To
Hell Router Tableish.

Now we move into the style you are enquiring about.
Personally I like to avoid these at all costs because the
user soon finds out that they work just fine and they never
get around to building An/The Ultimate Router Table.
Anyway, four sides (no bottom required if the top is
attached) is all that's needed. Really only two sides need
to be all the way from the underside of the top and the
surface it sits on. The other sides (usually front and
back) can be just rails to stiffen things up and allow
access to the motor/switch. One caveat, be sure to make it
tall enough to allow your longest bit to retract below the
table. The down side to being too tall is it may be
uncomfortable after its placed on top of a surface at
34"-36" off the floor. In reality many people find higher
surfaces more comfortable to work at.

As for side to side and front to back dimensions, it need
not be any bigger that it need be to take the router. Even
something 12" X 12" would work depending on whether or not
your router handles will fit. If you go the small box route
be sure to implement some sort of means for clamping the box
to the table in case you ever do anything large that might
scootch the table across the bench/saw top. The small box
also insures that you won't have any sag from the motor
though depending on how big your overall top is you may have
sagging on the infeed/outfeed sides. It doesn't sound
though that your table will be mistaken for an aircraft
carrier so you'll more than likely be Okee-Dokee.

That do it for you?

UA100, builder of An Ultimate Router Table (The Tom Watson
Model) in between things like getting his Powermatic 141 14"
(pre-made in Chiwan by children/prison labor), finishing up
the 2002 kitchen remodel, eBaying old tools for fresh tool
cash and general day-to-day life stuff...