Hi Walt, I emailed you back, but it bounced.
I looked through my decal stash for 5/16" checks, either blue/clear or yellow/clear. I don't have anything that large at all. I have some black/clear, white/clear that are 1/4" and some yellow/clear that are1/8".
I wonder if an RC shop might help you out; they need those big decals more often. I usually build 1/72th and 5/16" is gargantuan for that.
Another thought, if RMS doesn't help you out, try going to hyperscaleand ask around in either Plane Talking or Plane Trading.