Variable number of filter channels in Simulink

In Simulink, I am modelling a MIMO system. P inputs, Q outputs, and all possible (P -> Q) impulse responses are known. I could simply implement P x Q FIR blocks, and everything would normally be fine.

The problem is that I need to run the model, then modify P or Q, then run the model again. Multiple times, changing P or Q each time.

A similar problem occurs elsewhere in the same model, with vector-array addition blocks in which the i/o sizes need to be variable.

Currently, the model uses a Matlab function block, for which P and Q are inputs along with all necessary signals and coefficients.

Thank you for any advice !