I am using the strc one made for the SC10:

http://www.amainhobbies.com/product_...tor-Set-Blue-2

It is actually slightly narrower than the plastic one that AE sells for the b4.1.
I shimmed it and it works fine.

I would have never spent money to try it but one of my AE plastic ones broke on race day and I had the STRCs lying around. So I thought, "what the heck! Lets give it a go!" In any case, I finished 2nd in the A-main with them so I guess they're fine!