For me, it's more about the time that you put in more than anything. I make sure to work the left just as much as I work the right.
Ok, it's a little crazy, but here's what I do:
I've rigged a practice pad on my treadmill.
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What I do, is I'll put on my Beats headphones, and I'll walk (usually at a slow pace but for a long-ish time). I usually listen to country music or 90's rock while I walk. I only know maybe 4 rudiments, so I'll do a different rudiment through different parts of the songs. For example, I may do a single-stroke roll through verse one and then paradiddles in the choruses. Then, I may do doubles during verse two, and then go back to paradiddles during the chorus again. I might do flam-taps (alternating right and left on the taps) during the bridge. Or I might do singles through the whole song. After 30-45 minutes of this, you should have a pretty good workout, and chances are your forearms will be pretty tight...oh and you've gotten a little cardio in to boot! It's more about working out both hands/arms equally, and by default, you left arm/hand will get a nice workout. I never do "just left" exercises.