Hello,

   I need to cut a rectangular hole of about 10x12mm on a plastic
 piece of 90x20x2mm.

   I need to do it on 30/40 pieces each time, which is either not enough
 to start a session on some kind of cnc/laser machine or is not cost effective.

   The answer should suggest an economic method that can be executed
 in home, with commonly available parts, and that should'n require
 too much time to perfom.
 
   The resulting hole shouldn't be perfect but should look
 aesthetically good.

   For example, cutting a round hole with a drill and then
 "squaring" it with a flat file is not appropriate because
 of the time required and because two successive holes will
 hardly look the same.

  Thanks in advance.

dwery-ga,
     TRY USING A ROUTER WITH A WITH A SQUARE JIG AND A 1/8" BIT. YOU
CAN PLUNGE THE BIT INTO THE CENTER OF THE HOLE AND THEN FOLLOW THE
JIG. THIS WILL GIVE YOU A NICE FINISHED HOLE WITH LITTLE TO NO EDGE
REFINISHING. IF YOU NEED A SMALLER THAN 1/8" RADIUS IN THE CORNERS TRY
A 3/32" BIT IN THE ROUTER.

GOOD DAY,
airman33860-ga

1) you need to specify what kind of plastic
2) the problem may be easier if you can allow a small radius in each
corner (you will probably have to debur the edges with a file and if
the radius is small enough you can also square off the corners easily
and cleanly)
3) for something of that size to look "aesthetically good" means
certain criteria are going to have to be extermely accurate - you need
to specify more (does the opening frame an other part, is this a
machine guard or a piece of furniture etc
 
have you tried making a guide template and using a router with a small bit?

Easiest way is to get someone else to do it because there is no easy
way that I know of.

As mentioned, the material type is relevant. Square punches are common
enough and are drawn together with a bolt. If you can?t punch, and
must have a cutter, they do exist. Perhaps called ?Watts cutters?,
consisting of a twin offset geared axis so the cutter circumscribes
(or squarecumscribes?) a square. Never come across rectangular ones
but one can imagine the necessary geometry. Would cost megabucks to
have one made but one could design and make with milling machine, lath
and a bunch of small gears etc.

Other alternative is laser cutting. Many companies now do a lot of fab
with lasers, usually in metal but some specialise in plastics.
Horrible shapes are now a doddle, often transferred straight form
drawing CAD output to machine. Might be worth asking for a quote --
you might be pleasantly surprised at the price.

Best