If you visit this page of the website with the best archive of Herse bicycle photographs, and if you look closely at how the handlebar bag attaches to the stem, you can see what I hope to end up with.
If you are looking for a modern version, you should visit this page on Compass Bicycles to see their decaleurs.
I'll clean up the joints and get this off to the chrome plater next, and with luck it'll be the crowning touch for the front of the bike. It mounts beneath the custom Herse stem on its twin bolts. You can just see the holes in the second photo. They're covered by masking tape in the first photo.