Here are a couple of F Extensions I have made for Bb Horns.  The extension fits into the A/+ valve to give the player the open notes of the F Horn, so it is easier to pop out a low C or G without having to use the 1&3 combination, or 2&3 and the stopping valve.  With some various cross fingerings and judicious use of the hand in the bell, you can also use it to play some of the lower notes that are not available on the Bb Horn (those being Bb down to F#).  Bb to Ab are pretty easy, but G and F# require some pitch bending and are not the most reliable of notes.  A Bb with an F extension may not be the best choice for playing Shostakovich 5, but for the Beethoven Sonata, and a veritable host of other chamber and solo works, it is definitely a contender.

The first pictures are an extension that I made for a Yamaha Bb.  I made it asymmetrical so it would fit better within the bend of the corpus.  The first photo is the stock instrument with the stopping valve sleeve in.  The second is the F extension installed.  As you can see, the extension just replaces the A/+ valve slide and sleeve.  The third is a view of the entire instrument with the extension.

These are pictures of an F extension that I recently (May 2006) finished for a King Bb.  It is the same basic design as the one above, but I made this one symmetrical.  The brace on this one is forged out of 3/16" brass rod and formed to match the stock King braces that I used between the slide legs.  I had to bend it a bit differently than the one on the Yamaha extension because of the shape and to clear the overly large grease rings that King used on the ends of their outside slide legs at the time.  On both extensions, that brace is there because without it, the tubing in the extension would vibrate for several seconds after you finished playing a low C.  It also greatly increases the rigidity of the slide, hence preventing damage from twisting.