As an alternative, you may consider creating a jig that will guide a narrow high speed router bit around the shape you desire to cut out. This is an incredibly messy way to do the job, requiring the chassis to be empty to avoid contamination from the metal shavings the router will produce.
Last and probably most practical is to have a small local machine shop use an end mill to cut the shape. If done at low speed, this could (depending on the chassis material) be the cleanest method. If they do it on a computer controlled mill, you'll pay for some programming the first time, but should get away cheaper for subsequent jobs. As always, the more parts you do at once, the more economical it will be.