The project I'm doing is TIG welding two 3/8" mild steel square bars on an angle to a 1/2" square bar. The drawing is easier to understand than my explanation. Top bar only is illustrated.

the right angle pipes/angle iron, c-clamps, and vise grips I'm using for a frame just don't get the angles right.

Could I safely use modeling clay or something like that to hold small pieces at an odd angle just to get the tack weld made as long as I keep the clay away from the weld?

holding pieces.jpg