This one be very careful if you want to pursue it before I finish this all up. Already leaking with one type of silicone. RTF High Temp silicone is keeping hold and tight. Seeing some leaking through the layers so Ill recommend high resolution printing methods like resin for less air gaps. PP should also be considered for its material properties.

A simple way to reuse old 40mm heatsinks and bump up their cooling performance with water. Most 40mm heatsinks although aluminum have fins and threaded rods meaning they are just oversized versions of CPU coolers. So lets put them to serious work. So far Im going to recommend RTF Silicone High Temp for sealing up the inside chamber and around the fittings. Fittings are 1/8" NPT 4 push fit. Yes the water will be restricted more than larger hoses, but in reality you dont need that much cooling overall. Ive run a hotend on 4mm hoses and the block stays nice and cool. Not much else to say about this one. Simple enough.

Heatsinks used are 11mm tall, 40x40 otherwise. The heatsink should slip right in and be just about flush with the back of the heatsink water camber. Seal all internal cavities and right around the heatsink before fully pushing it in. Use a pair of snips to cut off the heatsink fins near the water ports. Try and have the fins aligned along with the flow from the inlet and outlet so their is less resistance coming in and going out.