Hello Everyone! I have been working with Brass and other metals for a while but I am running into some issues when soldering large pieces.

What I am trying to do is make a box with the dimensions of 5.5X3.3X1.5 inches out of five pieces of brass. Because of what I am using it for it needs to stay square and have tight tolerances.

I can solder the four sides just fine with no distortion but I run into a lot of issues when attempting to solder on the large bottom plate (5.5X3.3 inches) . I get huge amounts of distortion on both the bottom plate and the sides. The metal starts to warp and bend.

I am currently use a map gas torch with a large bushy flame and the brass sits on a few soldering bricks. I try my best to meet the metal evenly but it seems that no matter how slow I go I still end up with some levels of distortion.

I have tried several different thicknesses of brass and different kinds of solder.
if anyone has any suggestions on how to keep the metal from warping of how to better evenly heat I would be very happy to hear them!
Thanks in advance!