There is no doubt, at least in my opinion, that a GI canteen needs a cup and stove. Tactical Taylor makes good stuff, and several years ago a friend was working with them for a special design mag pouch to fit on the stock of an AR-15. This was for a medium sized law enforcement agency, not necessarily a huge order. They were working well with him, building several prototypes. So you may have good luck.

Side note, we visited the TT store 'bout a year ago, place was full of GI's buying gear before heading to the sandbox. It really irritated me that US soldiers have to spend their own money (and TT gear is not cheap) on stuff the gov should be providing...