Replace bottom tubes of Marzocchi fork with rigid tubes?
It wouldn't be a "simple rigid tube", because it would have to include dropouts and brake mounts. And it would also have to fit into the existing fork's crown. Where would you get such a thing? I don't think Marzocchi, or any other manufacturer, makes it. If you're handy with a welder, you could fabricate it yourself - but at that point, you might as well just make a whole new fork.

Just replace the fork. It's not difficult. Make sure that the crown-to-axle distance is similar to what you had before, otherwise it may screw up the handling.