Lyman GPR is your best option - I'm a lefty with a rock-lock GPR .50. They are great for patched round-ball, and i've bumped off a few deer and one groundhog with mine.
The other option, if you get a military-type musket, is a flash-guard on the pan...meant to contain the flame so the guy to the right in the skirmish-line won't get the breech flash in his face. However, with this option, i'd still recommend a surplus pair of ESS goggles...if you covet your eyebrows!