For my third frame build I made a small rack attached to the upper seat stays. Think of a TA front rack that bolts to a Mafac brake but brazed to the frame and behind/under the seat. The rack was made from K&S brass square tubing (use what you know and can get, there was a hobby shop across the street and I did a little slot car stuff as a youth. But what did I know about metals back then would fill a book). I used 56% silver for the rack joints and the SS attachment. Even with the lower temp silver I saw some sagging of the square shape here and there as I got the brass square tubing too hot.

The rack worked well enough for my small tool kit, the spare sew up toe clip strapped to the seat rails. Until it didn't and got bent askew during a crash. I have only attached a rack directly to a frame once again since. That time the rack was round steel tubing. Andy.