I've had this problem and tried several different ways, supergluing it to the sump, with sealant, without...

I think the key is not to overtighten, which is where I was going wrong. On my last (and successful) attempt I didn't use glue or sealant, but tightened it up with a 10mm socket on a screwdriver attachment. Previously I had used a small ratchet.

It's been on for 18 months now, and no sign of a leak