I know what you mean about the wheel past the corners. I've learned my lesson after the first two cut-outs for displays.
Nice and easy, moderate/slow speed on the dremel, a steady hand and stop way before the corners. After stopping go back and cut a wee bit more at a time until the hole is met.
If you secure your box/enclosure, you can use two hands to hold the dremel for added steadiness and control.