Ok, I finally got around to cutting the trackball hole...here's what I did. I know, probably could have done it a LOT easier (lol), but it worked fine nonetheless.
- Measured trackball plate hole diameter = 82/83mm (Highball trackball plate)
- Got an adjustable hole saw (The 'T' bar kind with slidable blades, adjusted to suit an 82/3mm hole). Cut the hole out of a scrap piece of MDF (MDP piece needs to stretch the depth of the lexan - front to back..to be clamped later.
- Insert trackball plate in CP and lay lexan on top of CP (precisely) - Clamp everything together (again, precisely)
- Turned everything over so bottom of CP/Trackball plate hole is visible
- Marked (marker pen etc) the trackball plate "hole" around the inside so you now have a circle on the lexan.
- Un-clamp everything and remove the lexan
- Line up the MDF piece with hole in it, EXACTLY around the 'circle' just marked out on the lexan, and clamp making sure to be precisely alligned
- Drill/route a hole through from the top (big enough for flush trim bit to slot through)
- Insert router with flush bit and flush trim the lexan back to 'match' the hole in the MDF piece that is clamped underneath.
- Un-clamp MDF piece and lexan
- Ensure CP has trackball plate inserted and test lay the sheet of lexan over the top - If everything went to plan, the hole in the lexan should now perfectly match your tracball plate hole
- Grab an ale to celebrate this seemingly easy yet somewhat head-scratch inducing conundrum...well for me at least

I haven't bullnosed/chamfered my hole (sounds ominous) yet, but do plan to. I'm still looking for a dremel type polishing wheel to give it a sheen, once done. Although I came across a great UK site for power-tools etc (
www.tooled-up.com)...maybe they will have something similar?
EDIT: the piece I cut in the pic in my above post was scrapped. My above method used a seperate piece alltogether - Just so folk don't get confused.
Craig