There are dozens of wonderful methods to create a caterpillar rearing box or enclosure. It’s wonderful to recycle and reuse containers for that purpose. If you’d rather not make a rearing container, you can easily purchase an inexpensive rearing container.
To make this box, simply use any sturdy box. This box is simply a shoe box.
A hole should be cut into the lid and some type of mesh should be glued or taped over the hole. You can use a piece of material (that stained t-shirt), a paper towel, a coffee filter, a net, or anything along that line. Caterpillars will need some ventilation. It can become too humid in the box.
The box is covered with a plastic grocery bag, the same as you’d place a trash bag in a trash can, but the sides folded back and taped to the bottom of the box. Frass and old leaves can be removed as needed. To remove old leaves, you can simply pick up the caterpillars and place them on the lid while you clean out the box. Afterward, you can add new leaves to the box and replace the caterpillars back into the box. If the bottom of the box needs cleaning in a way that just emptying it into a trash can won’t do the job, the bag can be removed and another clean bag placed in the box. No washing, no mess.
When they pupate on the lid, just take out the plastic bag and you have a box in which they can emerge. The bottom will absorb the meconium and the sides are cardboard so they can crawl up the sides if they fall.
They can be released outdoors the day that they emerge or the following day. Butterflies do not eat the day they emerge. They can be watched and enjoyed indoors without food for a day.