bafffle - restrictor cones
If your non pressure radiator is losing fluid and your only remedy is to open the top tank and install a baffle to knock the water down WAIT. This handy dandy cone shaped brass tip can be cut and sized to fit over your overflow tube. Slide it down the tube inside the neck. Its a low cost way to install a baffle to prevent fluid loss and requires no invasive surgery.
Three are pictured but you are only buying one.