I recently purchased a manually focused webcam at a yard sale for $3. As I played around with it, I discovered that it was very hard to focus it. The little lens holder was too hard to grip, and my hand always got in the way of what I was trying to focus on.
So, I came up with this simple fix. Take a cap, like from a soda bottle. Drill a hole the size of the inside diameter of the webcam lens holder. Use CA glue (Crazy Glue or SuperGlue or the like) to glue the cap to the lens holder.
Now you have a larger, more easily adjusted knob to turn to focus your webcam.