I worked in industry with a couple of types of home-made water jacket heatsinks.
Both require flat surfaces (ie machined flat) but you don't have to machine the surface yourself. One was just hardware store rectangular alloy tubing 2" by 4" with thick wall (1/8") and mounted vertically, the ends sealed with plugs and silicone and water hoses added. Because of the big internal diameter you can just attach it to a water tank and let thermal convection recirculate the water.
The other smaller type was just a block of alloy 5/8" thick and cut in a square about the size of your hand, then a couple of holes drilled through it the long way with little hose fittings screwed in. You can also buy PC watercooler blocks like this these days.