When I do an expensive project, say a table top, I make a caul the size of the glue-up. But you're right that it can get expensive if you make a custom caul for every project. If you work with the same thickness substrate all the time (say 3/4 inch), you can make some supports that can be used to hold up the outside of the top caul so that the caul doesn't dome over when you pull vacuum. You can make these supports out of failed veneer projects, or from test projects you ran. Stick your project into the bag, put your caul on top, and then put your caul supports around the outside and under the caul. If the supports are one veneer thickness different than your project, it won't make any difference. That will allow you to have a few cauls that you can use for odd sized panels.
Interesting question. I'll be interested to find out what Tod does. Maybe he has a better solution.
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