I've put a loose foil cover over the illuminator at EY, where John took the Lexan out yesterday.  The official dust cover for the lexan slit that we do have, D0902791-v1, doesn't fit tightly over the slit, so flops back and forth leaving a gap, so not an improvement.  That part is on -v2 (D0902791-v2), which I believe will fit tightly, and modifying -v1 would be a good short-term fix.  However, the better plan is to beef up the dust cover into a part that fits into the slit, and thus holds itself in place, which can then be secured with a couple screws.  In the mean time, I was uncomfortable leaving the slit uncovered, and folded a piece of foil to prevent something small, like a screw, from falling through the slit and hitting the viewport.  I called John and talked it over, so he's aware of the very temporary solution.