I think I may have been the "merino-suggester"!
I doubt a sweater would "felt" sufficiently, especially something that was machine-knit. The trick is to get somewhat finer strands in an overall thinner layer.
If you know a serious knitter (not an Xmas sweater-from-a-kit knitter), asking that person to obtain merino and knitting a tight rectangular piece that can be "felted" is not too challenging. Only a friend or spouse would do this, as it is less interesting for a knitter than sorting a bag of mixed US cents is for an Ancients collector. However, knitters often create sample swatches to practice techniques, so a little charm may do the trick.
I often take coin photos with background knitted pieces made by my knit-master wife; the color range of fabric
can really complement a coin in ways that the standard
tray colors do not.