How can I make a home made dark-field filter for my microscope with a fixed condenser?

Nope. The limitation you have discovered is typical. I ume you are placing a central round block on the condenser lens. I have used this same method as a demonstration in a college cl I have taught for years. As you said, this allows enough light to come through around the edges of the condenser lens to produce a pseudo-dark field effect. But to get enough light properly focused at high magnification you need a dark field condenser, a sort of prism designed for that purpose.