Micro metal boxes are pretty poor boiling containers as the volume is hardly worth the effort. Winding a bunch of tape around a larger metal box to seal it is good, but all that tape will have to be removed to somewhere else before use as a cook pot.

I went to an easily waterproofed plastic box for my kit. I generally have a metal cup or cook pot in my shoulder bag.

Larger kits I could make are in either a Trianga aluminum mess tin or a stainless Japanese Picnic box which need to be sealed with tape.....and as yet have not been put together.

The rectangular Trianga mess tin is very nice and I use it as a catchall box in my shoulder bag....the plastic boxed kit is in the bag too.