We use a rubber sealing washer never had one leak yet.
You mean you have never been back or never had a fuse blow!
Even with the gaskets water is going to ingress into the conduit.
I prefer to use 4x4x2 or the next size down of "Gewiss" box or equivelent make. Conduit entry left and right and flex from the bottom via a TRS gland.
What light fittings are you using? It hugely depends on the fittings as to how the final connection would be made.