365Astronomy Female T-2 Dust Cover with Female M42 x 0.75 Thread - Full Metal Construction
This is a T-2 dust cover with internal M42x0.75 thread.
This will protect your products such as correctors and field flatteners (any accessory with a male T-2 thread) from dust and dew when not in use on your scope.
There are also plastic versions, but we found that it is somewhat more difficult to screw on the plastic one, especially in cold and dark.
It is also easier to cross-thread the plastic version so there are certain advantages in using a full metal cover.