[DeTomaso] Transmission viewing hole cover

[Christopher Kimball]

I noticed on a friend's Pantera a square, plastic cover on the rectangular hole on the top of the transmission.  The hole exposes part of the clutch, and mine has always remained uncovered.  I have two questions:

1) how important is it to have the hole covered, and

2) where is the best place to acquire one of the little, plastic covers?

Thanks,

Chris