I'd definately switch to a #2 or 3 tip--probably the #3. The #1 tip
is too small for the size of your piece, meaning that it takes so
to heat up that the copper will oxidize before the solder melts. This
sounds like the problem you are having.
Also, in my own experience, I prefer to solder something like this
on a solderite board, without surrounding firebricks. Sometimes I
find that the reflection of heat gets in the way of where I'd like to
place the torch.
If I may be so bold as to offer a couple other suggestions (my
students have been working on hollow box construction and some of the
issues they ran into are fresh in my mind)....keep in mind that it
be more difficult to heat your bottom than your sides, as the sides
are easier to reach with the torch. Therefore, I would tend to focus
my heat towards the bottom sheet, or suspend the piece between two
bricks and start heating from underneath. When the flux starts to go
glossy (handy flux--a borax based paste), I heat from the top side to
bring the rest of the work up to temperature and to help minimize
oxidation at the joints.
However, with such a thin gauge bottom in copper, you may have
trouble with warping, so I would not suspend the piece. Generally, 18
gauge or heavier, especially in copper (it is so soft) may make life
Just my humble suggestions....best of luck!