No worries -- you can still be a materialist. Many worlds is the materialist solution to the problem of random collapse. (But I think that's what you just wrote -- sorry if I misunderstood something.)

Suppose that a particle has a perfectly undetermined choice to go left or go right. If the particle goes left, a materialist must hold in principle that there is a mechanism that determined the direction, but then they can't say the direction was undetermined.

Many worlds says that both directions were chosen, and you happen to find yourself in the one where the particle went left. So there is no problem with something outside the system swooping down and making an arbitrary decision.