Double overhand shrugs with heavy weight. You can do this in one hand as well (suitcase DL).
Try farmers handles. I made my farmers out of iron pipe from the plumbing section in Home Depot. They have held near 300lbs a piece without bending and cost less than $30.
A rolling thunder handle would be great. IronMind.com has them.
I heard Kaz say once that "there is only one way to be able to hold that much weight in your hands and thats by lifting big weight in the gym everyday."
Perfect advice!
Suitcase DL, double overhand DL, double overhand shrugs...
Throw your straps in the trash if you use them. my grip is one of my best strengths, i do all my rows and chins with a thumbless grip. i haven't used straps in years and i use a lot of chalk.
i don't train forearms, i just train my "grip"...farmer's walks, static holds, plate flips.
Good luck mate, i know you will do well.