Hi,
What I do is cut the thicker enclosure plastic with whatever I have to hand - hacksaw blade or dremel.
Then use a much thinner more easily worked plastic to create small surrounds for the screen, leds or buttons, its allows me to be rough and quick with the thicker materials and then precise in the lower cost, thinner, easy to work materials. Its also a good way to recover an enclosure if you suddenly find you need one more LED and your spacing is now out, just drill out the whole area and use a thin facia piece for the new spacing.
Duane B