Hi James,
I have been in contact with Ian at
Jobu Design (who makes the Black Widow Light Weight gimbal head) and pre-ordered one of their new Black Widow Jr gimbal heads. Yea... I know... I'll be the Guinea-pig but it will only be $230 and they will have a batch of 25 ready within a month. According to Ian, it will be good for more than a 300/2.8.
Cheers, Jens.