I have had good luck with factory gaskets-----be sure things are clean when you put it together.
Also, check for nicks or other surface defects on the sealing surfaces.
Once I thought I had a leaking mag gasket, but it turned out to be a small square cover just below the mag-----you might check any nearby leak sources.
Are you talking about the cover plate or the hall effect pick up?
The lightspeed hall effect modules always seem to leak, it's the seal in the housing.
If you are talking about the mag cover plate only, I have had luck by cleaning both surfaces real well and then using a light film of automotive gasket sealant on the gasket.