Congratulations on the new Forester!
Do a search in the Forester and Sunseeker Forums as we discussed this back last fall when many of us first got ours. Many have done what CreativePart has done and gone with the metal dually extensions, it's the best, permanent and costly. Can be purchased from Borg tire supply (YourTireSupplyShop.com) Duallyvalve Kits for Class C - Ford GMC 16-19.5"
or the Tireman in Ridgecrest,CA (Tire-Man: Home Page
. Both have online sales.
I'm changing mine when I replace the tires in a few years to avoid paying the $100-$250 for mounting and rebalancing.
In the meantime, I have gone with a flexible wire mesh covered inner dually stem from Pacific Duallies.
Did you know that the steel wheel liners come off like hub caps with a screw driver or small crow bar?