At Thomann you can order customized cases in all sizes you order.
I did this several times for unusual combinations of equipment and was always very satisfied with the case I received at my special order.
For my FC-12 I use a custom case that I ordered at Thomann back in the late 90th for another foot controller which fortunately had an appropriate size.Can you recommend a special case? Have you ordered one for the FC-12?
A Thomann standard case that might fit your needs might be this one :Can you recommend a special case?