how do I model a diaphragm at CL of support when there is a gap in between beams?

Diaphragm definition has a requirement that 50% of the diaphragm thickness should be within one beam.

If you have a gap in between beams that cannot be satisfied if CL of diaphragm is at the CL of support line.

Therefore if there is a gap in between beam ends you need to 1) shift the diaphragm from CL of support such as 50% of diaphragm thickness is within a beam (see screen shot below) or 2) define two diaphragms side by side each of them 50% within beams of each span.