If you're keeping your plates face down on flat surface, like a table top, you are exposing the up-face to the weather and shielding the down-face...so the plate will curl. Try flipping them over and waiting for a day or so to see if they flatten by themselves (if left too long, they might continue to curl in the opposite direction and actually reverse). So keep an eye on them, and when they flatten, instead of keeping them flat face-down on a table, tip them upright against a vertical surface (or hang them with clips from a stretched string, like the Spaniards do). The point is to have to freely expose both faces to the air. When you finish working with them instead of laying them flat, tip them upright or hang them.
But a very likely outcome is that if you build your guitar in the outdoors in a changeable climate, when you bring your finished guitar indoors, especially to a period of dry weather, the guitar will probably crack or distort. You can bet on it. And curly maple, especially if it is flat sawn is, among all the guitar-woods the most reactive of all to humidity changes. I don't know where you live, but if it is rainy and warm in the summer and cold and dry in the winter, your guitar is likely to become a small time bomb waiting to pop.
I know this will happen because I am from Puerto Rico and all the folk instrument makers work outdoors. But in Puerto Rico, the tropical weather doesn't change during the year: it's always warm and steamy. So after they are made, the folk instruments remain fairly stable. But when travelers bring those water-soaked sponges to the Northeast United States with its long, cold, dry winters and artificially heated apartments, the instruments invariably shrink, twist and crack dramatically.
