@trevorflowers for example #alphacool has the distributor block and the tiny waterblock I'd intended to use...

They even have a chonky 120mm radiator with 4 ports so you can easily [partially!] fill such a passive phase changing loop from the top...

Just please don't use water for a phase-changing cooling system because even if you keep it sub-ambient, the vapour expansion to 1700x volume will either make it not work at all or potentially explode the hoses in your face like some high-pressure hydraulics line...

But for the use-case of a design-first case, a phase-changing termosiphon cooling is propably the most effective unless you'd choose to custom-build a giant-ass heatsink with heatpipes like the trashcan Mac Pro, tho that'll likely be way too expensive to even prototype!