I've read so often that a simple plumber's propane torch (mine is
an Ace hardware variety) is more than adequate for soldering bezels
onto sterling sheet.
I used one of those torches for years and in my experience you should
be able to do the piece you've described. I've used Hard grade solder
on brass and silver buckles about the size you've described and while
it's not a trivial exercise --that type of torch obviously has it's
limitations-- it certainly can be done.
As you've surmised the main problem is getting enough heat into your
piece for it to reach soldering temps.
Simply stated you need all the help you can get. Open-air setups like
the third hand aren't going to do it because so much of your heat is
being whisked away by the surrounding air. If it were me I would set
up a "mini-kiln" arrangement where three firebricks meet, as if to
form the the bottom corner of a box. Elevate the workpiece slightly,
say 1/4 inch off the surface of the bottom brick so that the flame
can get underneath. Make sure there is a little space all around the
workpiece so that the flames can get in and around the whole thing
(you don't want trapped or dead air). The idea is that you're
creating a semi-enclosed space that you can pump heat into instead of
losing it to the surrounding air.
Keep your torch in motion to bath the workpiece in flame. By keeping
a careful eye on the colors --dim the lights if you need to-- you can
see where the "sweet spots" are for positioning your flame and which
spots are less efficient. Be flexible, go with what's working, but be
determined too because you've just got to pump the heat in until
everything comes up to temperature.
You might also consider using a charcoal block to set the workpiece
on. They heat up nicely and can help get heat into the bottom of
Also, consider a second torch. When I was doing large pieces like
this I had a second propane torch on standby, just in case I simply
needed to pour in more heat. Yes, this is cumbersome and all the rest
of it ... but I'm wearing the buckles I made using these techniques
so it can work.
For reference my buckles were as heavy as 14 gauge, often 60 grams or
more in total weight.
One last thing: it's probably going to take you a while to get enough
heat into the piece for everything to flow nicely. Watch your flux
to make sure it hasn't all burned off by the time things get hot
enough. If it has then try using more. If that still doesn't do it
then consider using a more robust flux, one that can stand a little
more torch time. Don't forget good old plain borax as a helper for
the job. The gods were smiling on silversmiths the day they gave us
in The City of Light