I use the 3Racing Out Tuned Diff ( absolutely the best F1-diff IMHO). With a F103/F1-09 diff axle you can cut it as long as you like and then use spacers to increase the rear width.
That said I'm running the front about 15mm wider than the rear. This makes the car feel more stable and it gives a little less front end bite - I've found this ideal for most outdoor tracks.