The dimensions I saw on some parts site are:
1.5" High
8.6" Wide
7.8"
I made a rough simple filter to measurements I made on the holder, didn't write those down, but the above numbers sound close to what I did, I'll bet I did 8.5 and 7.75.
Pleated furnace filters are typically 1" High and one could cut so that they preserver two finished sides, then figure out some way to finish the two cut sides - someone, maybe this thread said they had done just that. I think one of these would work fine and cost not much, less than $5 if the rest of the filter was thrown away. I'd make a second one, or buy a really big filter and make 4 Subaru filters. But, who keeps a car long enough to use 4 cabin air filters?