If you just have one jar with a few small caterpillars, you can dispense with the bin. You still need to create a moat of water to exclude spiders, and you need a way to catch the frass.
A "chip and dip tray" from Dollar Tree is the ideal solution. It's 15 inches in diameter.Place your small jar in the center compartment, then pour in some water to create the barrier to spiders.
The rest of the tray will catch the frass. You can vacuum it out daily with a "dust buster."
Caterpillars never leave milkweed food as long as it remains edible, so you don't need to otherwise contain them. However, occasionally one may fall off the stem. If you leave one milkweed leaf on the tray, the fallen caterpillar will wander around the tray till it finds the leaf. Check the lone leaf daily and return any fallen caterpillars to the main stem.
This tray could be used to display all stages of the monarch life cycle. Caterpillars representing each of the five instars, plus an egg and a chrysalis, could be placed in each of the seven compartments, along with labels and a leaf to keep each caterpillar happy.