Managed to get a suitable sized hole cutter for my clutch window (92mm) so I got straight in there and gave it some revs on the hand drill.
Perfect, the hole is centered and the porthole sits really well over it. I have bought stainless M4 x 20mm socket cap screws with flanged nuts and a tube of heat and oil resistant RTV off ebay.
here is the clutch cover holed.
now i'm going to paint the cover in the charcoal metallic grey, so using grey RTV.
will post up the final version when its complete.