I have been using the Michael Froden fly system tubing supplied by Guide Line for this type of tube fly. Here in the States I get it from one of the site sponsors
http://www.flyfishusa.com/. I glue a size small piece of tubing inside a large size piece of tubing (you need tubing 2 sizes apart) using Loctite 414 a superglue designed specifically for vinyl. Leave about 3/8" of the smaller tubing sticking out one end of the larger tubing and cut to a length that leaves the other end of the larger tubing free of the smaller tubing for about 1/4". This quarter inch can be used to seat the hook eliminating the need for a piece of junction tubing. Tie the fly on the larger size tube only and when done slip a cone head on the smaller tubing up against the shoulder of the larger tubing and heat the smaller tubing exposed end to seal the cone head on the fly. No glue necessary and this particular tubing does not seem to break where it has been heated. The Loctite 414 superglue is the tricky part. You can only get it at gear shops, i.e., shops that specialize in gears, hydraulic hoses and the like for heavy equipment. You want the 1 oz. bottle which will cost about $18 USD. Store it in the refrigerator when not using and always tap the nozzle clear and you will be able to use the complete bottle without it plugging or drying out.
Regards,