Please don't take this the wrong way but from your description of it having a little Dandy scale, etc. you sound like my wife packing for an overnight trip. LOL
To me, a kit like you want to put together would want to be as light as possible and as small as possible but still allow you to reload in a survival situation. A container of powder - as big or small as you think is necessary and a dipper to go with it. Dippers are easy to make out of empty brass casings and allow you to be pretty consistent in grain weight - thus eliminate the need for a scale. A set of Lyman 310 handles and dies for the caliber. Smaller yet would be one of the Lee classic loading kits. Everything you need to load a cartridge. If you need to resize your brass - then one of the old Lyman whack a sizers. Primers, cast or purchased boolits and a quantity of brass. Keep it size and keep it light and as small as possible. If you feel the need to be able to cast - a very small dipper and lead ingots small enough to fit the ladle so you can melt over a wood fire. A single cavity mold. To size, one of the old Lee bullet sizers that you use a mallet on to drive the boolit through. Forget the mallet in the kit - a thick stick or similar will work on the sizer. And if you think you need heat to melt lead - then most folks would have a package of waterproof matches. Matches are fine but a good flint and steel striker with char cloth and some flax tow will give you a fire every time.
A box is not going to be the handiest thing to carry if you are on the move. I'd be looking at some sort of shoulder bag, fanny pack or other soft sided zipper bag that would stuff in to a back pack if you have one.
I always keep going back to my muzzleloading experiences from the last 50 + years ands putting together a hunting pouch. Less is better. So many people carry so much stuff just because "they might need it". You have to ask yourself "what is really necessary". Example: Suppose your "grab gun" is a revolver or semi-auto. How many loaded cartridges will you actually have? Will you be able to retrieve your spent casings if using a semi? If you include empty cases in your grab bag, then you can save a little space by using a case such as the MTM with primed brass in it - seal it to keep it water proof with some tape. If you carry a case of 50 pistol casings that are already primed, that's one process you wouldn't have to do and 50 primers not taking up space in your kit. If you carry a MTM case of 20 bottle neck rifle casings - prime them - the same applies.
It's an interesting concept to ponder over and I'm sure there are as many opinions as there are people. And I'm pretty sure that if the need arose that you used the kit, you'd find things in it you would toss and things you'd add. Water, food, shelter, heat and a means of obtaining those things should dictate the necessities as well as what conditions you may find yourself in.