I like to use T-Pins to hold things like this in place on a soft
Solderite board or firebrick.
As far as not melting something, remember that whether soldering or
fusing, whether it is AS, SS, or copper, the two pieces of metal
need to reach the same temperature. Therefore, the larger piece
needs more heat. Use the flux as a temperature indicator. Generally,
when two items are this different in mass, I put the flame only on
the sheet, and let the wire pick up the heat from the "sides" of the
The thing to remember when doing something like this with Argentium
Sterling or gold, is that these metals do not transfer heat the way
that traditional sterling and copper alloys do. So, do not "heat the
whole thing". Instead, work sequentially, heating an area of the 20
gauge sheet until it fuses with the bead wire, and then moving to
the adjacent area, then to the next. After fusing one side, do the
same on the other side.
I hope this is helpful!