I agree with Scott that keeping minnows in a cheap aquarium has been ideal for my minnows. I keep them in my front window ledge to keep the water cold, but not freeze up and use an aerator to keep the water moving and every day I do a partial water change.
I should add that I also have the aquarium glass covered from the sun as not to get an algae bloom started.
This has worked for me and I don’t remember the last time I have lost a minnow from this.
Just my $.02