What's the biggest job you'd still use the 6 inch knife for? I'm guessing tight spots or repairs, but maybe there's a size limit where you just have to switch.

Hardware store fixes are classic, like using duct tape for everything.

Oh, that's such a familiar story. My great aunt's crock was the same way, a total mold factory. I've had great luck with the basic plastic airlock lids from a brand called Fermentology. They're cheap and they just work.