Cut a piece out of a plastic bottle about 1/2 inch larger on all sides than the slice you are patching. Clean it and rough up the top surface with sandpaper, then use superglue to bind the roughed side to the inside of the tire. Make sure you have the slice closed and lined up as best as you can before gluing, so maybe run a few drops of superglue inside the cut and bond the sides together before gluing the patch on. Wait 10 minutes, then reinflate tire and ride. The plastic won't be as flexible as the rubber, but it will give some, and if you sand the edges of the plastic too, then chances of tube puncture are also reduced. Also the pressurized tire tube will help hold the patch in place from the inside. I had to do this when I got a 1 inch sidewall cut in my road tire. Sidewall cuts are much harder to do though, and you have to be VERY carefull riding it when patched. But if you have a sidewall cut, you shouldn't continue riding that tire anyway.