I have replaced the glass in the fender mirror with part store bought convex pieces. I take the mirror apart and replace the glass (polished metal) with a part store bought 3 3/4" convex piece. You can stick them on with double back tape.
It may not be a good as modern mirrors but it works a lot better.
Speedway sells mirrors for oval track car they may work on a Cobra.
910 56855 - Speedway Motors, America's Oldest Speed Shop
910 56849 - Speedway Motors, America's Oldest Speed Shop
Some Cobras like the E-M, the dash is lower than the trunk (or the trunk is higher) so the stock mirror is to low. When you look into the mirror, on the bottom half you see the bulkhead of the car. You can raise the mirror 2" with a piece of aluminum that matches the size of the mirror base. Looks good and does not stand out.
Mirrors are a problem on a 60's English made car!
Dwight
Have fun and see ya on the road