I am joining some tongue and groove panels together for some cabinet doors. I have made some white oak braces that hold the panel flat and compress the joint while the glue dries giving a tight joint and flat panel.
I made my jig big enough to accommodate a 27 in. panel, but this can vary in size depending on your needs. The length of the dowels can vary too depending on how many panels you glue up in tandem or the thickness of your panels. The rails are 1-1/2in. x 1-1/2in. with 5/8 in. dowels to define the closest width. I then drive wedges between the panel and the pegs as I tap down the holdfasts. You can incorporate wax paper or parchment paper in between the panel and the jig if you think the glue will squeeze out and stick to unwanted surfaces.
I put in an even number of wedges to give equal compression. I drilled the holes in the jig on the drill press and then sawed them in half. I would not want to gamble with the holes not lining up. The wedges have to be narrower than the panels so the holdfast can completely flatten the panels.