Look around in the junk or visit the friendly neighbourhood aluminium fabricator and you could carry home offcuts of rectangular section aluminium extrusions. I got a good length of 2.5 x 1.5 inch section used for door frames/partitions etc.
The first step is to cut off two pieces of 2.5 inch length. Mark off the mid point of the 1.5 inch side and make saw cuts, so that you now have four U-shaped pieces. File the lengths of two pieces so that they fit within the channel of the other two pieces. The accompanying photos make the procedure clearer.
Do note that heat dissipation depends on the area of the active surfaces and on air cirulation, and also on the colour of the heatsink-- black radiates best. To help with this, make saw cuts on the sides of the U-channel, and using a pair of big pliers, slightly bend the fins to stagger them. Again, the procedure is made clearer if you refer to the photos.
Assemble the four pieces in the order shown and drill the necessary fixing holes to mate the heatsink to the LED holder. Mine needed three holes and another small hole for the wires. Mark and drill one hole carefully, keeping the alignment of all the pieces. Now clamp them together with a nut and bolt and then drill the remaining holes. De-burr the holes, clean all the pieces so that the surfaces mate closely, and then paint the 'free' surfaces matt black. Now dab some heatsink compound on the mating surfaces and screw the heatsink to the LED holder, carefully passing the wires through its hole.
Your home-brew heatsink may not look 'spic and span', but it surely is a "cool one" when it comes to doing its job well!
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