Here is a transition from 5/8″ OD/ 1/2″ ID vinyl tubing to 3/4″ OD/ 5/8″ ID vinyl tubing to Amico 5/8″ OD 2M Black Soft Corrugated Tube Cable Pipe Bellows Hose.
Both 5/8″ OD pieces slide with some resistance, well into the 5/8″ ID vinyl tube. These connections are air tight and relatively secure, with an orthodontic rubber band on the 5/8″ vinyl piece that comes out of the melodica. The rubber band rolls into the larger tube. One might be advisable where it slides into the melodica.
The total length from the melodica opening to the end is 14.5″. Lowboy recommends 12-14″ for his technique (“the Bootay Technique” I call it). I am 5’9″ in height, and it will have to be shorter than that to position the melodica over my torso. As I have modified my Hohner HM-26, only the top third of it will have to make contact, with the melodica held in a vertical position, cradled at the bottom in my left hand, to get a wah-wah effect.