Is there a decent sealable bag you can decant tinned stuff into for boiling?
its worth investing in a cheap vac seal machine, you can get external vac packers for about £20 and the bags work out 20p each if you buy 100 and then I just cook my chilli, stew, curry ect at home and then bag it up, vac seal it and freeze it, my freezer is full of meals like this cos if there is any left overs from normal dinners it gets vac packed, i pay a bit more for the bags that you can use for 'sous vide' its a form of catering that uses hot water baths to cook and reheat foods so they are tough enough for a good boil and no chemicals leach out.
then i just have a rummage and pull out my required grub and the fact its frozen means you can use it to keep other stuff chilled until it defrosts in your pack.
also if you pack it while its hot and give it a good seal there is a good chance that it will stay safe to eat, so long as its well reheated even if its been defrosted for a while.
cheapest grub i can find