Hi,
I have spent quite a lot searching on this subject quite recently. I manage tu put the HERTZ DSK-165.3 6.5" set in the front. This is component set which is coming with tweeters and crossover. Those speakers are really big, and I struggle to intal it. The best what I found are those spacers:
Subaru forester 1 - dystanse MDF Komplet Warszawa (3686880433) - Allegro.pl - Wi
Unfortunately they are not available in UK. I try a lots of different places. The above web page is referring to Polish 'ebay'. I will in Poland for Christmas and will coming back in the middle of January. I am more than happy to buy it for you or other forum members. The cost is ridiculously cheap - max 10£ per set

I ma traveling with car, so I can take few of those if anyone want and the re-send here in UK.
Oh one more thing, it is much better than plastic because it can absorb some vibration and they are design in such way that speaker will tilt and point more towards the driver. You will probably need to re-drill mounting holes, but still I think is worth.
If anyone is interested in those, please let me know, before. I am more than happy to make a list order.
Oh, and one more thing, you can try with Rockford Fosgate 6.5" (coaxial-not the best solution because you probably have a stock tweeters, so ideally you want component set but anyway will be much better than stock). When I comeback to home I can re-check model I have. You might be able to install them without any spacers, because they are really thin, although not the cheapest as well. They coming with a plastic mounting brackets, so there should be no problem. I used 4" Rockford in the rear door, and the plastic bracket work well.
Look at that page, is quite useful as well.
http://www.subaruaudio.net/installations/02-08-sg/
Lucas