Copper really sucks heat and the surface oxidizes fast when heated.
When copper is the larger mass, as is this case, you are dealing
with a difficult situation. (It would be easier if the box were
silver.) You don’t say how large the piece is, but the bottom line is
that you have to get a lot of heat on it to flow the solder. If your
copper box is larger than an inch square, you may need two torches.
and maybe so even if the box is smaller!
I use slabs of kiln brick to build an oven enclosure with an open
front, that allows access for the torch. The oven holds the heat. To
somewhat control that oxidizing problem I have had success with a
heavy application of flux (I use Battern’s) on the joint. Also, I
’tin’ or sweat solder the easiest silver solder I have on the silver
part (Argentium-easy flows at the lowest temp). Then hit the whole
thing with lots of heat, as fast as you can, keeping the torch on
the copper. As soon as the solder flows, take the torch off.
My soldering silver onto copper is relatively small (cuff bracelets)
but long, so the oven approach is the only thing that has keep the
heat high enough for the solder to flow.
Judy in Kansas, where Roma beans are coming on, but the tomatoes
have quit. for now.