How can I strengthen my hands so I can play the marimba with four mallets?

I need a quick way to strengthen the sides of my hands where my ring fingers pinkies are. I'm a high school percussionist and I've never played with four mallets for more than eight measures. The parts I've been given for this year's marching band show require me to use four mallets for most of the show.

By the end of the first percussion rehearsal (which was today), the outside halves of my hands were so tired and weak that I had trouble putting the cover back on my marimba! My band director told me it's normal for my hands to hurt because people don't usually use that side of their hand to pick things up. He said I just have to get used to it. I don't know how long that's going to take, but I want to get used to it fast. I'm the leader of our front ensemble and when percussion camp is over, it'll be my job to make sure everyone learns their parts and knows what's going on. I can't really do that if I can't even hold my own mallets correctly.

Also, I don't know if this matters, but I'm a small person (4'11") and I have really small hands. It's really hard for me to get a good grip on the mallets and I don't know if that's because my hands aren't strong enough, because I'm not holding them correctly or because my hand are too small. The mallets I hold with my ring finger and pinky keep flopping around everywhere. PLEASE HELP ME! Thank you! :D