Couple of ideas:
Either you could make a flat piece of nuno felt then cut and sew yourself a hat (i.e. use the nuno felt as 'yardage')
Or, just add fabric to your top layer of wool when you make your hat - it doesn't have to be a complete covering - it can be interesting to just have fabric here and there.
If I were making a hat over a resist, I would choose a fabric that nuno felts easily i.e. a loose weave.