Good stuff guys.
Geo, if you need to make a camera port transparency for a Spitfire (or Mosquito) in 1/48th scale, then the average paper punch, used for punching holes in sheets inserted in loose-leaf 'ring' binders, is ideal. Just punch out discs from thin clear sheet, and the 'holes' from 'solid' plastic sheet, cutting the latter to the size and shape of the hatch - works perfectly.