Is there a motor heat sink without a fan that is going to fit with the rustler transmission... and actually work to bring the temps down?

Wow, surprised nobody recommended the fastlane transmission case. It's not a heatsink, but it is solid aluminum that will dissipate heat, and open up the motor area to let it breathe. Only problem with that is it leaves your motor out in the open and prone to getting beat up on unless you get yourself a swami bar or like what I did before my swami bar arrived. I got an rc-monster motor heatsink which is aluminum finned and looks cool as all hell, until you use it as a motor guard. BTW try disabling your punch control if you can stand it, and put your start power to high. See if your temps come down after a pack or two.