What they have now is 30mil. I think with printed vinyl on it & it's shrunk almost 1/4" all around.
that is alarming, a 12" x 18" having 1/4" shrinkage.
How old are the magnets?
Don't know how old they are, we didn't do these for him. See attached photos. The black around the edge is the mag material where the vinyl has shrunk.
your standard magnet price and then the shop hourly fee for contour cutting, .
or buy them some scissors and let them do it