I helped a guy to make a collector box for an Interstate 1500 once. We got a length of light gaugestainless steel pipe (it was free), cut it to the correct length, cut a slot each end and fitted it. Two auto style clamps over the slots secured it.
Not the proper way I know, but 6 years later its still there and sounds the same as with the old collector box. Off the line performance seems the sam, not sure about top end as the owner never takes it over 80mph anyway.
I still have some of the pipe, I'll measure the inner diameter later.
