Technical details
The implementation of this idea is quite simple. All it takes to try it is to remove the cork from the bocal and glue on a slightly conical (outside) plastic tube, with the smallest end towards the bottom of the bocal. In this tube there are 4 grooves, where rubber rings should be placed as needed for the bocal to fit tight in the socket of the bassoon. With each tube a set of rings in 5 sizes is supplied, to accomodate the various dimensions of Fox, Püchner, Yamaha, Heckel etc. bassoons.

In use
Mount the smallest ring first, in the lowest groove. Check the fit, it should not be too difficult to push the bocal all the way down. If it feels too loose, try the next sizes until you feel it's right. After this, apply the other rings as needed. Most often the biggest rings are used at the top, the rule-of-thumb being that the ring size should follow the conical shape of the tube and socket.

It is adviseable to grease the rings a bit before inserting the bocal in the bassoon, in order not to stress them too much. They may break if the fit is very tight and you force the bocal. Just run a finger across your face where it is a little 'greasy' and apply this to the rings. Twist and turn the bocal gently to its end position, also when removing the bocal after playing. If you don't stress the rings too much, they will last for years, so finding the right combination of rings for your bassoon is not something you should do every week.

TIP: from experience, Fox owners may have to leave the bottom groove empty, no ring, as the space is too small for any ring to fit. The hard plastic makes the connection and seal instead, which is just as good.