I would recommend a high speed cutter (i.e. #115 or #196) instead of
the cutting discs for this particular application. See this link for
available tips:
http://www.dremel.com/HTML/products/accessories/carving.html
I would also recommend using the dremel tool to cut an initial hole
much smaller than your intended opening, and then doing the final
sizing with a hobby knife or CAREFULLY with a Dremel... it's much
easier to start small and then open it up rather than starting large,
and then having to fill in with putty or sheet styrene.
Good luck.