Have you tried a thicker
Have you tried a thicker fiber paper and adding more weight behind and on top of the dams?
You can put some shelf strips across the tops of the dams (like bridges) and weigh them down.
With sufficient weight the only way the glass is going to run is if it seeps through the fiber paper. I know lead crystal is very viscous at those temps — I don’t think it is that thin.
Helios Kiln Glass Studio