Though I have no experience with an engine coolant-to-steering-fluid cooler, a regular old air-to-power-steering fluid cooler (which was just a an extra loop or two of steel steering pressure line passing through a rack of brass or alloy fins) used to be a very easy thing to score at a junkyard - especially if you looked on a cop car, or sometimes a heavy duty towing-package equipped car. Now, I haven't looked for one for a generation, but they were pretty common back in the day, and easy to plumb in as they mounted between the pump and it's usual hoses. Since they were intended to cool with the air already having been through the radiator, they were probably no more efficient than the coolant one, as the steering fluid would only be 'cooled' when it was above the temperature of the pre-heated air... but they didn't do what this one can do when provoked, I suppose.
Alternatively, there used to be coolers for transmissions available in just about any size, and since they rarely had thermostats or bypasses, they were pretty cheap - though the packaging would definitely be 'other-than-stock' looking.