Robust control tuning against uncertain FRD plant

I have nominal, upper bound and lower bound on my plant as SISO FRDs, that is, complex value vs frequency. I built a usys from the nominal + uncertainreal*(upper - nominal) with uncertainreal between -1 and +1. I use a tuneableTF for a controller. The combination via feedback is apparently a genss that can't be tuned because it contains the uncertain FRDs. How can I do this? That is tune the TF controller against the uFRD plant? I really really don't want to fit the FRDs with a TF.