I don't know what you are finding, but the envelopes of dry milk that I've seen over the years make into quarts of milk.

See if you can find Ziplok bowls with screw on tops. They come in several sizes; one may suit your needs. It may help to keep the powder out of threads if you remove it with a spoon or small measuring cup. (Doing the same may allow zip bags to work.) Tying non-zip bags, as someone else suggested, came to my mind, too.

We all need to find what works for each of us. I hope you find your system soon.