| If I've understood what you're trying to do correctly, the easiest way to hold the perforated tube in place at the tailpipe end would be to get a couple of short lengths of SS tube, the inside diameter of which is just slightly larger than the OD of the perforated tube. Weld the two short lengths of tube onto the back of the endcaps, so when you fit the endcaps the ends of the perforated tube slip into the 2 short pieces you just welded on. That's more or less how it's done on the Termi etc race cans.
As for diameter of perforated tube, it's pointless to increase the diameter unless the diameter of the tailpipe is bigger. Try to match the perforated tube diameter to the tailpipe diameter.
A16 should be able to supply everything you need.
Just my 2p worth!
Cheers, Neil. | 
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