The "U" is somewhat important (see below). The rest of the obstructions that you have don't do anything to help the cooling, because they don't act as fins: they're too far from the heat source. So what they do is restric flow, that's all.
If you're going for multiple jets (7 jets?) then the "U" shape isn't needed at all, except for the structure, i.e. making sure that the top doesn't collapse, and/or that the baseplate of the block doesn't bend under the mounting pressure.
So if you remove that "U" and replace it with 4 small posts, you might be better off. You can either mill those posts, or you can solder them on, it doesn't matter.
It's also an opportunity for you to reduce your machining time. You might consider building this block in 3 layers:
1-the baseplate (copper)
2-the outside wall (acrylic/polycarbonate,...)
3-the top (also plastic)
and bolt it all together, with RTV (a silicone sealer).
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