Any Advice on a Cup?
Yes - Always drink from the side with the rim nearest to you or you’ll get a wet shirt!
Seriously though, when you say ‘boiling cup’ I take it you mean the steel mug that comes with the BCB set?
If you use BIB meals, you eat from the bag and brew with the boil water.
Why isn’t that sufficient?
Only if you are using the mug as a ‘kettle’ to decant from (to share the water) can I see the need for another cup.
If that’s the case just use another steel mug - The Dutch ‘44 stainless pattern or the Austrian stainless version of the Brit ‘58 mug are pretty robust and multi-function.
I’ve never seen the point of the insulted mugs - the heating up of a beverage and having it hang around is counter productive - the idea is to get it down yer neck quick to get the warmth inside you.
If you’ve got sensitive lips or kiddies using them a bit of gaffer tape on one edge of the rim helps.
And wrapping (gloved) hands round a hot mug is another heat transfer/morale advantage. Win-win!
That stated I do see the value and use the .3 litre ammo pouch flasks (double wall/insulated) primarily as a flask with a pre made beverage but which doubles as a drinking vessel. Win-win.