Held in place partly by friction, partly by panel nails. The design is far from perfect (the braces are probably slightly the wrong size) but it’s more sturdy than tape.
Also, I scraped off some of the duct tape glue crud off the window.
We should probably make a proper box type thing for this at some point!
There was a problem last summer with door readers not working when the weather got hot.
The space gets sunlight most of the day on the doors and the units heat up.