jim:
Why not determine a convenient airframe with a -21 installed and procure a timed out cooler? I think a King air C-90 had them?? It would also provide a nice Chinese blueprint for mock-up and install.
Also, if I recall, you could add the -25 and -27 to the airframe list which includes Twotters and PC-6s, plus the -11/-15s found on Air Tractors. I believe all these dash numbers operate 80-100psi engine oil systems with the same oil tank capacity. But check the specs to confirm.
If you're affiliated with a school, someone may want to donate the necessary parts with the right persuasion...
Good luck!
W1