I have ordered a set of track day additional mufflers from ACT. I spoke to ACT before making the order and they will tweek the dia of the fitting to suit my pipes as it is under a mm or so different to their standard size. But looking at their web site they will make the fitting size anything you want. 2.5 inches is 63.5mm and my pipes measured to 63.8mm. So for 58mm you would simply need to ask them to make them to fit that size. Hope that makes sense.
I also want to avoid drilling holes to use a retaining screw over and above the exhaust clamps. I will wait till I have the parts and then see what alternatives can be done but one idea I had was to use lock wire if something suitable nearby to wire it too.
