STi8 Interior in a Classic
 

I know it's been done and I know the rears also fit with a bit of a shove.
But what do people do about covering the rear (boot side) of the STi rear seat, as from memory this is just exposed foam due to the Sti having a solid rear bulkhead.

..
 

early wrx's have a thin plastic cover that slides behind the seat to hide the foam, that should do for you....