It's just a piece of screen and some scrap lumber. I cut everything to size on the table saw then put the frame together using brad nails and glue. The screen is folded over at the edges and stapled. The door hinge is a piece of leather and the door is held shut by a neodymium magnet that attracts the screw holding on the door knob. A light coat of danish oil finished it off. The entire project only took about 30 minutes.
In the image below, you can see the magnet countersunk into the frame. I drilled the hole to be snug and coated the inside of it with super glue before pressing in the magnet. I've done this before on several projects. The metal holding the knob on actually ended up working great and was more than enough for the magnet to attract. Here are the magnets I used:
There is a little green praying mantis in there. As I was putting in the last staples and testing the door in my garage this bug jumped in. I saw it all happen and just stood there in stunned silence. There aren't words I know of for coincidences this crazy. I could have searched my yard for hours and not found a bug this cool to show my son and here he is jumping over my shoulder and into our little bug cage just as I finish it. Perfect.
Here's a video of the enclosure in action. When my son woke up we studied and talked about the bug for a while and then went outside to try and let him go.