Get a bucket and a snap on lid, may want a bungie over lid. Drill lots of 1/4" holes in the bucket to half to two thirds way down and the lid. Tie a weight to the side of the bucket and a rope on the handle. Let it sink to the bottom.
Add minnows and drop off the dock. Have a small bucket of lake water in the fridge, change out every 48 hours and scrub bucket, no soap. Pour fridge water in minnow bucket add some minnows. Put rest of minnows in the bucket back.
Academy sports has a gated lid that fit 5 gallon buckets.