I have just about completed my second frame in two years using a MAPP gas - air torch. velonomade mentions reflectors in passing in the threads you've referenced. He suggests stainless steel flashing pieces but my local hardware store only carried aluminum. You burn holes in aluminum before you've finished brazing the joint. I took a steel 1 gallon paint can and cut part of the side out of it (about 40% of the circumference) leaving the bottom intact. I did it with a hacksaw. You've got to angle it for clearance as you go down. I've found it makes all the difference in the world. When possible I put the lid on and get the part to be brazed into the can. If it is not possible I use the can and the cut out piece to reflect as much heat as I can.

I'm sure that firebrick and/or ceramic wool would work even better since you have more control on the shape, but I had a can handy and it works.