Scribe your lines on it with a utility knife, then use your dremel to cut as close to the lines as you can. Then use a fine file to finish the slot. You can also just use a drill for the extreme ends of the slot (use a drill the same dia as the slot is going to be).
I use these wheels on my dremel - works much better than the standard ones.
http://www.amazon.com/Dremel-EZ456-2...3344771&sr=1-4
They also come in a kit
http://www.amazon.com/Dremel-EZ406-C...3344771&sr=1-7
Losi 8T 1.0, Savage Flux - XL style, LST XXL, Muggy, 3.3 E-Revo Conversion and sitting outside 425hp, 831 Tq Dodge Ram Turbo Diesel. It SMOKES