Here's a couple of hinge-making tricks.
Cut a piece of tubing the length of your hinge. Mark the knuckles,
and saw-- but not all the way through! Leave just enough of the tube
uncut so that it will hold its shape. Do this with each knuckle
*making sure the closed parts are all lined up on the same side of
the tube*. I usually apply wite-out to the middle knuckle and solder
the other two (or whatever), with the uncut part facing AWAY from
the joint. Then finish sawing, remove the middle piece. This is now
for some other use, as it will be too loose by twice the kerf of
your saw blade. Cut a new piece for the space in your hinge, making
it fit exactly right.
When I solder the other half of the hinge, I always do it with the
hinge assembled. Wite-out may be too thick to stay on the surface
where the knuckles meet (I don't like ochre for this as it may run,
and even the tiniest contamination will make your life difficult).
Old-fashioned carbon pencil works moderately well as a solder
inhibitor. But I always protect the parts that could get soldered by
accident. The hinge pin for soldering can be any high-melting-point
metal that is well oxidized, but if you can find the right size,
mechanical pencil "lead" will work, or, best of all, titanium wire.
It cannot be soldered, has a very very high melting point, and is
not a heat sink. It is very hard to draw down, but I'm here to say
it can be done. Come to think of it, niobium would probably work
well too, but I don't know as much about niobium.