For SON or B&M or others that use crimped or soldered connectors, unless the hole is big enough to pass the crimped-on ends, you'll have to crimp them after passing the wire through the BO, and cut them off to remove the wire.
If you're sure you'll never have crimped ends on your wire, then stop reading here.

My preference is the ones that are an open "C" instead of an "O", so the wire can be loaded from the side. To ensure it won't happen accidentally on the road, use two of those close together, open sides facing opposite directions, and only a bit more than a wire diameter between them. That requires you to orient the wire at 90° to the fork blade to load or unload them. Usually the two "C" partial loops are made of one piece of tubing so it's not so hard to get them oriented correctly to each other when brazing. Use thick-wall tube, since the open-C shape is vulnerable to crushing if you whang it on something in the world.

Sorry I don't have any pics of ones I've made, but such things are available in a larger size for hydro brake lines. This pic is stolen from ceeway.com (frame parts supplier in the UK)I have a pair of plastic ones that came with some Shimano dyno-hub, that attach with double-sided tape, not very elegant but could be made much smaller in steel. I've had the plastic ones on a utlility bike that's been ridden in the rain plenty, for I don't know how many years, 20 maybe? That double-sided tape never let go. Just yesterday I peeled them off because I was adding lowrider braze-ons, and they were impressively difficult to remove. Unfortunately my attempts to find them for sale were unsuccessful, so I don't think those are currently sold.

Someone with CAD skillz and a 3d printer could make the Shimano-style easily enough in plastic. Lemme know if you need pics so you don't have to reinvent them, but it's not rocket surgery. Just make the curved "foot" specific to the diameter of the frame tube you're sticking them to.

Printed in stainless for brazing would be awesome but I have no idea how to get that done. If someone makes 'em, please sign me up for a dozen of them.

They're pretty fiddly to make out of tubing with a hacksaw and needle files. Pro tip: braze the bit of tube to the fork (or downtube or whatever) first, then file out the opposing C-shapes and the gap between them. The bit of tube is hard to hold in a vise for filing. But then the stakes are high if you mess one up, so try it on scrap before committing to a real fork.