I have made one tube so far, and so far I am very happy with it. It is the augmenting type, and I made it from .008" 304 from McMasterCarr.
Bellmouths: I once found a formula that said the radius should be internal dia/5. This is what I used, and the proportions seem good. Bellmouth was spun over a mdf plug in the lathe. I use full length lithoplate aluminum outer duct, 4mm spacing at the rear, and 6mm extension beyound the inner duct. I am running a JG100 engine, so the dimensions should be ok with 120 class turbines.

Lars