Just fitted sti interior in my uk turbo 2000 now when I open the boot I can see all the foam on the rear seat what normally goes there to hide it

 

The Uk 2000 has split rear seats. So the back of them are panelled and carpetted.
There won't be anything standard you can buy, you could make something up out of mdf.

 

The classic sti never had fold down seats I am sure! So there was something behind the seat.

 

I have this having put a Spec-C bench in place of my WRX one - there was a carpet trim on the back of the arm, but there was a plastic/rubber trim behind the parts of the seat that were foam, attached to the bars that run from the parcel shelf to the floor.

You could probably get some flexible black plastic and knock something up.

 

There is ment to be a black hard plastic sheet that cover the whole of the rear bulkhead.

 

Did you get the cross-brace with your rear seat back?

When I bought mine, I got them from GradeA who told me that the UK cars have split seats,so are braced at the back, whereas the STi ones don't, so need this cross-brace that bolts across the centre. He showed it me and I removed it from the car myself.

With it came a hardboard panel and a grey cloth cover.