Something I have often wondered is how hard it is to do openwork, but without the modern jeweller's saw. Nowadays, we'd say fretwork, or piercing, but it's quite different to the medieval method, which either involved chisels, or casting.

https://www.metmuseum.org/art/collection/search/464956

I'm having a go with chisels. I am fairly certain that the belt fitting (above) is openwork - it could be cast in theory, but it doesn't look like it - the lines are good, but the symmetry is off.

As always, Theophilus is our first stop.

These examples are all in the Metropolitan Museum of Art, with a 7th - 8th century date. They are key rings, basically, hung from the belt, with tools, keys or ornaments hanging from the different loops. In some places, the wear marks are really clear. Copper alloy, often tinned.

I'm sort-of copying the example at the top of this thread. I'm using 1mm thick copper sheet, which is a cheat, because these seem to be 2mm thick, typically. But it's what I've got, so I'm going with it. I'm less worried about utility, more worried about developing technique.