3/8ths is no where near 11mm.
the decimal equivalent to 3/8ths is .375
11mm is .43307
thats a difference of almost a 1/16th of an inch. you NEED to get the correct mount otherwise it will slip over time, or you will grind out your set screws trying to make up the difference since it won't seat flush.
I went through all this suffering when I was trying to find a scope mount for my whisper (which is a 11.5 mm dovetail). no one makes the correct one-size-only, you can only get these kind that are supposed to fit a variety of sizes as mentioned earlier.
right now I am in the process of machining my own mount. I've been a machinst for fourteen and a half years now, and I've got it all drawn up. the only thing hampering the project is the dovetail tool, and the correct one costs $60+.
once the "stimulous" check shows up I'll buy the right one and get going on it.
-Mark