See here. Smallest water block for a 4090. Its proper good and whilst its not the coolest its still right on it and its a single slot.
I desperately need to read / watch more PC hardware liquid cooling reviews again. I had no idea that non-single-slot designs with waterblocks are the thing... good find @vestibule Any advice on where to look for custom loop cooling gear?
I cannot justify water cooling because of the cost. But for those with the money this is a no brainer. There are other reasons I do not WC and so I do not pretend I know any thing about it. The main reason I posted it up is because its terrific.