[Shiraz (Ray)]

Hard to describe in words, but here goes.
Determine the extremities of the focal plane that you want to keep clear of vignetting. For a rectangular detector, these will be the corners of the detector. From one of these extreme points, project a central ray back through the centre of the lens and out into the outside world.
Then, from the front of the lens, draw two rays extending out towards infinity, from each edge of the lens and parallel to the central ray. Do the same for the other side of the focal zone. The shield can be anywhere outside of the cone defined by the external rays.