What is a good alternative to a factory master cylinder? Maybe it's just the reman units I have bought but the last two I've had leak out underneath the lid.

Strange as it may sound I fought this issue several years back and Long story short, found that by tightening the spring latch (bending it) on the lid the problem went away
hope this helps

While you are waiting for it you might check that the MC perimeter and the lid & both sides of the gasket are clean/smooth/no tears/good contact. You might lower the fluid level slightly as it dont need to be all the way up near the top. that topping off is for your average Joe that might not check the level regularly like we enthusiasts do. The fluid would need to be very very low to be in danger of causing no pressure. Most levels are way higher than what is needed for a safety margin & that's not a problem as most dont leak