I ended up using boxes of books. I'd say there was probably 200 - 300 pounds across the 4 X 6 foot surface. I probably could have gone with even less weight, but the final product seems nearly perfectly flat.

I did my own experiment of gluing a flat piece of mdf on the edges of two other pieces of MD, using only 3 books as weight. The next day I could not pull the assemblky apart.

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