The lenght of the tube is up to you. I wouldnt go over 4"though. I cut a 4" x 1 1/2"blank. Using a really small drill bit like a 3/32, I drill the center all the way through till they meet up in the middle. Chuck it up on the lathe and turn the call down and shape it to your desired design. The mouthpiece end should be 1 1/4 diameter when done and sand. Pull it off and use a forestner bit to core the call. I use a 1" on the mouthpiece end which will give you 1/8 around the edge left. Drill that down about 3/8 "to 1/2". The small pilot hole you drilled before you put it on the lathe will guide you to keep it centered. Just keep an eye on it and make sure you dont get off center. Change to a smaller bit such as 3/8" if you want it to be baffled. Drill that through the body. Flip over to the bell end and using what ever sized bits needed, gradually step down the sizes to follow the contour of the bell. When you finish that , you can use a dremel tool to smooth the bell end out. For the lipstop, cut a half moon piece that when glued in place will leave half of the 1" opening open. Once you glue that in and sand everything down, your ready to slap the latex reed on and give her a try.
Dont forget in your design to put a groove for the O-ring to fit in and another groove if you want to hold a lanyard. One tip to remember while drilling out with your forestner bit is to often pull it out and clean off the shavings. If the call feels warm, give it a little chance to cool off. If it gets too hot, it may crack.
I hope some of this made sense and can be useful.
Travis