My uncle would clean the fish when we got home from the stream. he would take the stomach out and put the contents in a small glass with some water and swish the contents around. This would break apart the clump of partially digested and undigested food so we could see how small the bugs were that they eat.
I had a creel that was fabric with mesh ends to allow water flow. It had a ruler built into the side for easy measuring. I would hang it low so it would dip in the water. This worked well until I lost it during a move.