Because of the thinner boards, the problem seems
to be multiplied.
The edges are square, but the boards themselves being soft in nature
tend to cup under pressure.
I made this temporary jig and it's working great.
Don't let the "apple press" look make you laugh, it's only for
2-5 lbs of pressure or just enough to prevent the cupping.
It's all I could come up with in short order.
The board (next to the clamp handle) slides in a cut track under the
table to clamp any size from 20.5" to 0".
There are square markings on one side to make sure the boards are
squared up before clamping.
I'll be making a larger one soon and will spend more time on it.
I used 3/4" birch plywood for most of the jig.
The size of this jig can be according to the users need.
I drilled and mounted a second hole in the press board to use
for smaller panels.
If you need anymore info (text) I'll be glad to help.
These drawings were made on "powerpoint" so I had to transfer
the format to .bmp, so if they aren't clear, let me know.