I used to wrap the bit of tissue in foil, dip in liq. N2 then grind in a mortar and pestle.  I unfolded the foil carefully to extract the tissue powder (I needed it to extract RNA from the tissue).  I used a ceramic mortar and pestle.  If you haven't got one, ask Jon Holdich or Barney, as one might have turned up in the course of the recent lab clear-outs.  Hope this helps!  Lisa

Suggestions are fine. Foil will keep ground material together. The mortar and pestle is standard Chemistry small equipment for mixing chemicals.

Thank you for all your suggestions. Regards