Suppose I wanted several containers to store exposed 4x5 negatives, but I didn't have enough empty negative boxes. What could I easily buy or make that I could safely store exposed negatives in?
You might consider these items I found on website :
Delta 1 Light Tight Safe-T-Bags with Labels for Film and Paper (5-Pack)
Doran Paper Safe for Film and Paper 8.5 x 11"
They're the closest thing I can think of to what you might be looking for.
If by exposed you mean exposed but not-yet-developed negatives, then you can use any lightproof clean smooth box similar in size to your original negatives box. You can in fact use almost any lightproof box and pad it with clean velvet, microfiber cloth, lens cloth or Pec-Pads inside, so it doesn't scratch the film.
As an additional measure, you can put/wrap the box in a piece of black garbage bag — use the thicker 120/240 liter ones, they are much thicker and almost completely opaque. Wash it from the factory dust and dry it before use or you will end up with dirty negatives.
If you use 2/3 layers (depending on the quality of the material), you can make your own lightproof bag/envelope similar to those they use to package darkroom photo paper. Just make sure to do a thorough flashlight test in a dark so there are no light leaks or holes.
For proper protection you would probably want to use both the makeshift lightproof bag and the box. Which one goes in which is the matter of personal preference, but I would suggest you put the negatives in a bag and the bag in a box, similar to how the sheet film is originally sold.