Blog/2026-06-30/Modeling a Watch Strap Print with Claude
A short while ago, I bought a set of Apple Watch Ultra 2 bands from one of those undifferentiated sellers on Amazon. The style I've now come to like are the ones called the "Alpine Loop". It's a nylon-fabric with loops on the outside of the band. As far as I can tell the loops are intended to be entirely decorative. However, they provide a convenient location to attach an RFID tag and therefore let myself into my apartment building.
Most RFID tags are in a big gray enclosure that you then add to your keychain, but handling the keys while you're carrying your baby is an exceedingly mild inconvenience. Naturally it's a high priority to solve this problem. People do chip their animals and so on and you can't really imagine them doing so with a massive disc, so I looked it up and apparently they make these little tags meant to be inserted into animals. Some people put them into the webbing of their hands between the index finger and thumb. Those people are not buying from a random international eBay seller for $16 a pair.
Now that we know the form factor works, it's a matter of building a holder that fits into the loop with a bore to hold the actual glass RFID tag. I needed to fit the holder's hook into the loop and then have a little lip to reduce the chance it'll pop out of the fabric.
In the past, one might have needed to model with Blender or some CAD tool. But these days, with Claude Code and the power of build123d you can get quite far with just prompts. After prototyping a dozen or so, I was able to find the right sizes that worked.
Julie doesn't have the same style of strap since she prefers one of the velcro ones from the same set. The fabric loop she has is a bit looser so it's easier for the hook to fall off. So, for hers, I glued down the hook to the rest of the body to form a closed shape after passing the strap through it. I used my Flipper Zero to write the tags and then everything works fantastically!
One funny thing that happened is that while trying to get one of the tags off one of these holders, I accidentally snipped the back of it. I suppose I must not have damaged the antenna because it still functions. As punishment for only humoring me and not responding with wild enthusiasm, Julie has this half-tag.
I've published the script that will generate the 3d model here.
