Your word “holdfasts” lost me, so I went to the photos to see what you were talking about.
I went to a “aluminum” computer “scrapyard” when I was living in San Jose, CA; (1977-2004) I was looking for and found some thin perforated sheets of aluminum with holes about 2mm in diameter for my Korean tarmac, (runway) to put my new 1/32 F86 Sabre Jet diorama on. This place had everything you could think of, when it came to aluminum, so I bought several kinds of rod and bar/block size items. I was at the right place at the right time!
I left San Jose, California in 2004 and came back to live in Southern California in 2016. Maybe the scrapyard is still there !!! RJW!