Hi Ethan,
Fillet Pro is on my list of things to try out, but right now I just use Harris 50 and 56. In this case I used both. The binder bolt needs a nice fillet, so I use 50 there, and the 5/16" extensions into the stem piece get it as well to build up a tiny fillet. I'm pretty sure I used 56 on the 5/8" cross piece, the extensions pierce the cross piece and brazed at the intersection.
As Mike states, I make these for the "Ocean Air Ortlieb hack" style as well as a more traditional pin type that fits the VO handlebar piece. So many people have purchased the VO decaleur ($25!) that there are a ton of them ot there, but a certain percentage of the buyers have an issue with it (headset loosening, breaking, bag placement wrong), so I've been making those about 3-1 over the Ortlieb style.
I have some of both styles in my personal collection. A shout out to Rob at Ocean Air for a brilliant idea-it's dead solid, easy to remove, and you'll never lose your bag on washboard gravel.
Mark