Saw these on ebay, I know they are not the correct ones and not of same quality, but might be possible to utilise the red cover.
ebay 111496165208
(http://club.alembicguitars.net/Images/449/207903.jpg)
I actually have red covers for the collet knobs, which look similar to these.
What I don't have are the knobs from the original F2B preamps, which were more squat and had set screws.
Like these dusty babies?
(http://club.alembicguitars.net/Images/449/207951.jpg)