A strong second vote for the 20mm ammo cans... They also about perfectly hold the two 25# bags that make up a 50# box of bulk Fa powder (or two bags of Fg from two bulk boxes of that).
A more robust solution...
In 1970 when the Omnibus Crime Control Act shut us down to 5# buy/possess limits, I purchased and covered with a few layers of autobody undercoating a 16 guage 55 gallon drum with a removable head, sealing ring/gasket, and cam-type locking ring. We mounted it in a hillside by burying it and then poured a concrete collar around the top into which we mounted a steel round with a skirt welded around it. It was mounted with two hinge plates at the rear and two enclosed lock boxes at the front (the locks protected so they cannot be shot at or into). After lining the inside with wood ("non-sparking material"), we invited the ATF to come inspect our "magazine". Very unorthodox but we passed and obtained the permits necessary to continue buying 50# boxes of black. Right after that Paul Barnett and the Indiana Sportsmans Council had their federal senator introduce Senate Bill 1096 which passed unaminously and we no longer required the magazine to keep shooting. What a great "day box" for storing powder in though. It did require 75 feet of clear area from people, animals or inhabitable buildings in order to pass that inspection... but maybe less if you don't have to pass that...
A little more pricey solution than just the ammo can and probably recommended where rainfall is high...
With a little shopping, you can buy a lockable tool box for tossing in the back of a pick-up truck that will just hold the 20mm ammo can , bury the aluminum tool box, put the ammo can in it with powder inside, close and lock the tool box, put a layer of plastic over it (a layer of foam with the plastic will keep its temperature more "earth-stable"), and then cover it with a few inches of sifted dirt. Again mark your location so you can reliably know where to dig.
Good luck and be safe!
GOW/George