I checked the fit for a number of different lens covers and I found a pair that fit the Zeiss zoom eyepiece quite well.
They are the Vortex medium lens covers.
If you wanted to get a replacement of the original cover that came with your eyepiece, I would recommend you contact Zeiss to see if that part is available. Typically, manufacturers don't supply dealers with those type of replacement parts.
You can reach Zeiss at: 1-800-441-3005.