It Must Have a Name

When you remove the lens on a DSLR, you have a big hole that it is begging for dust and dirt.  Of course you always put another lens on it at once.  My previous Pentax DSLR had a black plastic insert that would fit into lens connector to keep dirt and bugs out while I was carrying it out to the telescope where I would put the telescope adapter in place of the lens.  It is not a lens cap.  The dimensions are wrong and it is rather more like a T-ring.  What is it called so that I know for what to search?

Camera dust cap.