I was hoping someone here may know the answer.
I had a full size bed at my appt in college and it did not have a middle rail going under the mattress. My dad ended up buying a piece of wood from the local hardware store and had it cut to fit and just placed that in the middle to give the extra support. We also have a full size conversion kit for our son's crib, but our kit actually came with 2 pieces of wood to go across the frame for extra support. You may want to go on the companies website to see if they recommend anything.