Creating a prefab for this, while I'm not one to shun such a request, is pretty trivial.
I'm assuming you're talking about nos_button.
There are two ways a person could go about doing this. We'll branch in the second step though.
1)
-Create the square block that will be the body of the switch. (I'm working with the texture dimensions for simplicity. Scale it down as needed.)
-Create a 32x32 12 sided cylinder (Again, scale as needed. More sides to the cylinder makes it smoother but at higher r_speed costs.)
-place the the cylinder over the the round part of the switch so they line up
-Ensure the textures are aligned. I do this by clicking on the block's texture that was fitted to the block and right clicking on the buttons face. Just make sure it's lined up.

2) Branch 1: If you
have to have the rotating switch flush with the button.
-Don't use carve.
-First make two cuts using the clipping tool to define the area within the button.
-make one cut across the newly formed square

-make a series of cuts to the points of the cylinder. Note: You may want to use vertex manipulation to ensure that the corners of the cylinder land on points in the grid.

-Now take each little strip within that square and it's a good time to practice vertex manipulation. Move each corner to a point on the cylinder. (Or you can keep using the clip too for each strip. Whichever you're more comfortable with.)

2) Branch 2: If you don't mind the rotating switch sticking out a bit from the whole block. EASIER.
-Instead of cutting all this out, just stick your cylinder out of the brush 1 unit. You don't need to cut anything this way. It'll look just as good, and use less wpolies as well.

3) Make the button a func_rot_button, ty it to a momentary_door adjusting values as needed and voila.
So there ya go.