Non-Scalar Structure with Different numels as Parameter in a MATLAB Function Block

I'm implementing a supervisory control function in a SimEvents model using a Matlab Function block wrapped in a Simulink function. It will be called by several custom entity servers to transfer data from a "grand plan" into the attributes of SimEvents entities. The function will manage the plan, recording dispatch status etc. in the same persistent structure that contains the data.

I want to import the initial grand plan from the base workspace as a parameter to the MATLAB function block. I have met most of the constraints for codegen in my code, but I am stuck on the data type constraint, which is only satisfied if all substructures have the same size. My structure is in the form of buckets and packets:

>> numel(bucket(1).orepacket)

ans =

   202

>> numel(bucket(2).orepacket)

ans =

   204

My baseline assumption is that I will need to pad the orepacket substructure in the workspace so that all buckets have the same number of orepackets. Is there a better way to deal with this? In the Ports & Data Manager, the parameter is tunable but the "variable size" checkbox is greyed out.