When I do flat objects that have a thicker edge than it's interior surfaces sometimes I get curling. I have found the best way to correct this is to (this is using the B9 curing unit) fill the glass container with water and place the uncured model under it so that the weight of the water and container flatten the print. I usually cure 3 min and flip the print and cure another 3 min. after this curing in water is not an issue.
Just wanted to share.