I've never really noticed the carton box taste, but I grew up with this stuff, so maybe I'm not a fair judge. The most well known brand is "De Ruyter" but if they are better than the others? I'm not sure... definitely more expensive. Some stores (like Aldi and Lidl) sell it also in plastic packages, but that is harder to contain, and I think that lacks a bit of taste... Well they are also cheaper....
Hot summer is definitely cruel to hagelslag. Since it's just chocolate it will melt and that can also influence the taste. It's not needed to put that stuff in the fridge (I recommend against it actually), but making sure it's not exposed to heath is surely needed. It would definitely be recommendable to keep it at a temperature below 25 degrees Celsius (20 is even better), and to make sure the package is not exposed to direct sunlight. When kept in a tent with hot weather, that stuff can definitely get "maimed".