I keep 5 bins going most of the time. I use the large sweater boxes made by sterilite. They are about 24"x16" and 6" deep. With the slick plastic sides the beetles don't get out. The bins are clearish. I don't keep them in the dark and they do well. I have drilled holes all around the edge of the lid to allow plenty of ventilation. Then I stack them. That keeps them out of the way. I rotate the bins so that the one(s) with treat size mealworms are/is on top.
For easy catching I put in plastic lids from mayonnaise/peanut butter jars. The 1/2"-3/4" lip is perfect for the larger worms. They seem to climb out of the bedding to pupate, so the larger of the worms go into the lids. With 3 lids in a bin I can dump a couple hundred in a container quickly to go out and see who wants to be friends.
Matt