I make that key lime recipe at home with bottled lime juice, no zest, and an 8 oz container of Cool Whip to make it creamier.  You can buy dry mix to make whipped topping and it will work on a canoe trip; works best if temps are not too hot.  Have also made the Jello cheesecakes and you can make a pie out of any Jello pudding box, just make it with less water so it is firmer.  I occasionally take a "can", well hidden in my pack, and a small can opener (or I think some condensed milk comes with a pop=top can lid)  :-X  I have also found that the Dollar Tree stores (and other dollar stores) carry regular milk in paper/on shelf cartons. Alternatively, if you repackage the condensed milk, make sure the container you put it in is sterile or boiled in water to kill bacteria and then keep it in the shade or cooler, deep in the pack, and use it within a few days.  It might be fine, there is so much sugar in it that helps retard spoiling for a while.
These dollar stores also have some other interesting items in multiple single serve packs: tomato sauce/paste, nutella, zippy cheese dip, peanut butter, etc. that work great for camping.   ;)