I have seen a factory p/s cooler about the same size as the Perma-cool unit you have installed on a '00 Mercedes-Benz CL500. I've also seen a '92 Honda Prelude with a factory p/s cooler that looks like this one.
Perma-cool's powersteering cool website is here.
What about running your choice of cooler, then plumb a filter into the return line. Navy Lifer sugguests in this thread
either ACDelco: P/N PF883 or WIX: P/N 58964.
How would a normal spin-on oil filter work in this siutation? Maybe use a remote oil filter mount
and a standard engine oil filter plumbed into the return line just before the line enters the reservoir. The plus side to this would be filtration of course, but also increased fluid capacity of around 1 quart. I don' think the line pressure is really an issue in this portion of the system. When replacing the filter, I thinking, pre-filling it with p/s fluid would help. Any remaining air bubbles would flow to the reservoir and escape through the cap.
Questions, comments, conerns on my ideas?