A recent update which is not documented yet might help in this case. If you have a look at Bridges and Frames you will notice that we place tabs left and right of the PCB, because of this we require a minimum size of 15mm by 15mm. However actually it is only required to have a height > 15mm, the width doesn’t really matter. Because of this we update the check to only validate the width.
What does that mean in your case? Just rotate the single PCB by 90 degrees and it should perfectly work out 
1 Like