Nice concept, I've never tried brazing either.
As for mounts, I've tried a lot of things over the years:
- P clamps are OK but are diameter specific and offset the bag, plus they're a bit floppy.
- For a while I made a hose clamp mount from a thin strip of aluminum with three M5 holes in it, and it worked great, but took some fabrication time.
- Then I tried the SKS Anywhere Mount, it worked much better than I expected, but still rotated and slipped a little after a while.
- Finally I designed a new improved version of the hose clamp mount, and had a local CNC shop make me a batch. Functionally it is the same as the earlier hose clamp mount, but I added recesses so the hose clamp body could be tucked in, out of the way. The test one on my commuter bike is working great, install and forget it.
At the risk of squishing your DIY spirit, here's a link to the CNC mount (note this is my own side business).
http://cleavelandmountaineering.com/product/fork-clamp-mount/Or, if you're going DIY on the entire cage, just integrate a hose clamp mount into the cage itself, for a lower profile finished product.