Great notes on choosing different copper pipes (apparently there's red copper and blue copper?) by @sara on this post:

https://publiclab.org/notes/sara/5-2-2012/materials-copper-pipe-work

Also a good discussion of whether adhesive copper foil could work, or if it'd interact with the acetone badly, with @gretchengehrke and @zengirl2:

https://publiclab.org/notes/gretchengehrke/11-02-2017/copper-sheet-hydrogen-sulfide-detection-activity#c17756

I was looking for some pre-cut smaller pieces of copper in standard sizes, in part to avoid the warping that cutting big sheets with shears can cause. I found these copper stamping blanks that have 2 holes predrilled:

https://www.amazon.com/RMP-Stamping-Blanks-Rectangular-Copper/dp/B00MHWRLDI/

They're $13 for 10, so $1.30 each, and the holes could make it easier to mount them.

But what's best is that they have a protective film coating -- designed for stamping through with letters and numbers. I was thinking that potentially this could be removed when you want to start the test, presenting a consistent surface at least the first time... but in any case preserving the surface even before you sand it down and acetone it as per Gretchen's documented method: (#15142)

They're 1"x2" with rounded corners: