Looking on line it appears Subaru has 56.1mm hub size. I could be wrong here, but all current Subie wheels will use the same size.
I found this:
http://www.prestigewheel.com/Catalog/HubRings66_67.pdf
as well as Mackin Industries that lists Glass filled polycarb rings, telling reader to call them for needs at 562-946-6820.
Also:
http://www.justforwheels.com/index.jsp
Looks like lots of info out there.
Glenn
If Bill's correct there won't be a ring out there that would work. If they're that close in size why bother?