I have an update on my project.

I have been looking at doing away with the welding and using ABS plastic caps and clips.

In the images below is what I intend to 3d print. There will be a 40x40 square tube running inside the block to supply water to the other side off the 200mmx50mm rectangular tube (water block) so hoses are connected at same end.
The 6mm flat bar will be held in place by the clip and a 16x16 channel either side clamped together with M8x110mm bolts, I figure 1 clamp set every 3rd PCB. Flat bar to be drilled and tapped and PCB secured by M4x10mm nylon screws.