I've repaired cuts up to 1/4" by using the Tyvek race numbers:
1. rough-up the inside of the tyre with sandpaper, like stuff included in patch kits.
2. cut up two pieces of Tyvek, nickle and quarter sized
3. use 3M Fastak Super Weatherstripping adhesive, I always have a tube around for sew-up tyres
4. follow instrucitons and lay down layer on both Tyvek patch and tyre, let get tacky.
5. add on more thin layer Fastak and apply patch.
6. Use handle of screwdriver to "roll" over patch to apply pressure
7. Apply 2nd patch the same way
8. sprinkle some talc powder to make excess Fastak not sticky
9. clamp lightly between two pieces of wood in vice for a couple hours
The Tyvek and Fastak actually transmits the load from the cut casing threads across the cut. You'll notice that the dark stripe in the picture at the cut is skewed into an S shape? With the Tyvek patch on teh tyre, that area will be straight.