Some time ago (see Vol. V, No. 3 this newsletter, Ed.) I described how I had installed a key on my bassoon to emit e-flat in the middle of the bass staff. Since then I have installed about six of these keys on various bassoons including my personal instruments.
I have varied the exact location, angle, and size of the tone hole slightly each time, and I have also varied the design of the mechanism, sometimes using Heckel parts and sometimes making the keys from odds and ends.
In a general way I can say that the tone hole must be considerably higher on the instrument than Heckel makes his d/e-flat, trill key. And the finger touch lever must be located so that it is operated by the 2nd or 3rd knuckle of the index finger (right hand).
The result is a stable e-flat - regardless of reed. The timbre of the note matches very nicely the notes immediately around it, and it is possible to slur to or from virtually any other note on the instrument. In short, it is a replica of the e-flat - b-flat key of the Boehm clarinet or the b-flat key on the saxophone; and it is just as convenient to use.
After having used this key for several years, I can report that it greatly simplifies the problem of the reed and makes it possible to tune the notes e and f (in both octaves) considerably higher than would be possible otherwise. I have now virtually abandoned the forked fingering for e-flat.
Some unexpected additional benefits I have discovered are:
I expect that this key will become standard on all bassoons in the future. It is long overdue (200 years??), and it is certainly more valuable than many of the useless gadgets I have seen on so many bassoons.
If and when more and more young bassoonists use this key I imagine that some more new fingerings will be discovered perhaps for the extreme high register.
May your low D not be too sharp!