Whats the best fitting to connect to a small undersink ariston water heater?

[Gary111]

Hi, I bought tap fittings but they have a bit sticking out so it won't seal.
Should I get a 1/2 inch female connector instead then use either the tape or a rubber washer?

Or are there some flexible tap connectors that would take the pressure and temperature for long periods?
That seems a lot easier but also reduces the bore to 10mm, maybe that's also a problem?
Many thanks