I have only used re-purposed material for one set: a well loved cutting board (more like "cutting
sheet") under scales on (one of) my 16 ('s). Used a yellow board, but the color really doesn't pop. I don't really care that much.
Most of my other liners were made from new shower pan liner (come to think of it, that
is re-purposed - I guess I was thinking recycled - since it's being used for an unintended purpose....) which is gray but has the added bonus of keeping the steel under the scales dry (so far) and adding a little shock absorption. Also used has been red plumbing gasket material, which is a little thicker.
My 17:
My only large Becker with liners, the 21 (with VZ G-10 scales)
This one has maple liners....

but they're epoxied to the katalox (Mexican Ebony) scales so...not such an easy project.
Honestly, I like the shower pan liner more than anything for its cheapness and extreme ease of installation. I also like its "just right" thickness for the tweeners and how it provides a little vibration damping (OK, I can't quantify that, but it
feels pretty good in my hand) as well as how it seems to keep the bolts from loosening up (I punch out undersize bolt holes for water infiltration prevention and grip) since it's hard to find stainless hardware that doesn't sit proud of the tweener scales.
Pic, but the project thread is
here:
I like liners on the tweeners, but not on the full size Beckers, for the most part - tho they work well on the RBK for me. FYI, the shower pan liner is on a roll at the Gnome Despot and costs almost nothing; it'll liner MORE than a few knives. I bought a 1'x3' piece (I think) for less than $10. I've given plenty away, used it for about a half dozen knives...and I still have some left for at least a dozen more. If you don't mind gray, it's the shizznit.