Of course, you’ll want to laminate a note like this. You have the options of heat lamination or the simpler adhesive kind. I find that I can do a decent enough job with clear packing tape and a little patience.
I’ve used this note several times. As the instructions on the .pdf recommend, be sure to warm the lid before using hot glue, otherwise the cold metal will harden the glue too quickly and it won’t stick. You can either leave the lid in the sun for a few minutes or use an open flame to warm the lid immediately before applying the glue
If you’re not too concerned with water damage (hopefully your ammo can is water tight) you can print these out on full-sized sticker sheets and then cut them to fit.
This .pdf document contains two stash notes.