Those Crystals are still hazardous waste, some chemicals that are not terribly dangerous as liquids can be extremely toxic as a dry powder that can become air borne and inhaled. A plastic rain barrel may not be made of a plastic that can handle chemicals. One really needs a chemical proof plastic container. Check with chemical supply companies, they may have containers that were used to ship chemicals to them that
hold larger volumes of liquids. An open container should have a cover, best would be a lid made of chicken wire over a layer of window screening, to keep kids and animals out of the container. Last thing you need is to find the bones of the neighbours kid or the dog in the bottom of your chemical storage tank.....
Now, most photographic chemicals should play nice together, and are not that dangerous, although concentrate bottles that have a little bit in them, should be filled with water before being added to a tank. I would keep any tank down to about 25L so that it's light enough to transport. If you really want the powder idea, then you should have a tank, with a valve that allows small amounts to drain into a large pan, say a drop a second, so that there isn't a lot of liquid to evaporate at any one time. The pan should be lined with a chemical proof plastic bag, that is large enough that once in a while you can tie the top, and take it to the hazardous waste depot.