I just used scrap aluminum, what ever was around. I think the thinner plates are 1/16th and the end plate is maybe 3/16. I cut all the disc's out on my band saw to 4 inch diameter. I hole saw drilled the thin plates to 2 inches. I made a template of the bolt pattern to drill the bolt holes. I stacked them and drill them all at once with a 1/4 inch drill bit. The washers are just some thin brass washers to evenly space the plates. The best washers would be aircraft thin grade, since there a consistant thickness and come quite thin. If i got serious about this I would lazer cut or stamp the disc's out or stainless. This baffle tunes like the supertrapp.
BTW, the spark areester was removed. Today I tried it with 2 more plates than shown and it wasn't much quiter than uncorked. As it is shown now the sound is quieter than uncorked but not by much, but still pulls like it's uncorked. If this buggs me to much I will either go thinner on the washers and add plates to keep the flow the same or remove one more plate and see what happens to the top end. Just as supertrapp
says the disc's are tuna-able. You know, supertrapps 4 inch disc has a 3 bolt pattern, wouldn't that be funny if there bolt pattern was the same a yamaha.