There are a number of pressureless casting techniques available. Investment casting is widely used, for instance. https://www.harmonycastings.com/ is a fancier example.

For this specific application, the manufacturing method determines the porosity of the material, and therefore the heat transfer.

CNC prototype parts will have better heat transfer than pressure die cast, and the pressure die cast will perform better than pressureless cast parts.

I wander what's the cost of (renting) metal 3D printers nowadays

Word of advice, never tool up unless you absolutely have no other options left.

https://jlcpcb.com/3d-printing/?gad=1

But if you must:

https://iro3d.com/#products

Metal printing has been available for awhile. =3

you sound like a bot

Text makes sound in your universe?

https://en.wikipedia.org/wiki/Synesthesia

Have a glorious day =3

I'd CNC machine that part, which is much more expensive in-quantity than the casting option but for a handful of prototypes gets you there quickly.