I needed to install an EZPass in my new-to-me M2C for my commute, and I found a cover that doesn't look too terrible. It is black silicone and slips over the modern small tag design. Installed it is about as unobtrusive as you can get. I put it in the corner on the passenger side of the mirror.
The kit I bought is on Amazon and is called "EZ Skin Bundle" sold by "IOT Slash" for $15. The kit includes a black rectangular pad to cover the exposed area on the back (that you would see from outside), along with all-black Velcro strips and alcohol cleaning pads. Before I installed mine I used a black Sharpie to black-out the tag areas that might otherwise show white through gaps in the cover.
