Cutting and drilling a Hyper 7 chassis.

[E-Revonut]

However you do it make sure that you drill a hole in the corners. 90 degree corners are weak spots asking for problems, plus the radiused corners look better. It's not a big area that needs to be removed so if it was me I would drill two holes, one at each end of the slot, pick a bit that is slightly wider than the spur gear and then use a dremel to remove the material between the holes