How can I join 20“x20”x1" panels into a one-end-open box?
Build a frame with tracks for the filters to slide into.
Use U channel for the tracks.
Get 4  60.75&quot; pieces of 0.5x1x0.5 U channel and cut notches in the sides at 20.25&quot; and 40.5&quot;
Fold them into U shapes
then join them together to make a frame.
Something like this Picasso.

the U channels can be connected together using rivets or welding or whatever.