If you're talking about the newer R6's, funnily enough I had just looks these up today. The MRA rule book mentions these two:

http://www.eti-fuelcel.com/R6G.html
http://www.traxxion.com/R6TankSliders.aspx

Can't vouch for either of them since I have an '03 R6 but figured since they were in my history I would pass 'em along.