Sugi ban advice

[Dauph]

My first attempt at a sugi ban box. I first tried burning boards, but they warped/cupped so much they were unusable. This time I made the box, dry fitted it, put a lump of concrete on top of it and burned the sides and left it to cool in the hope it would stay straight. It pretty much worked but, it had "unplanned side effects" on the joinery (I can make better boxes, honest!) After rubbing it down, the box joints are now proud. This is ok (ish) as the whole box has a very textured feel. I would describe the overall effect as "ahem" Rustic!
I would like to improve it though.
I'm using a plumbers type blowtorch for the burning.
Any advice, suggestions tips?