No matter what type of gear you intend to transport on your Subaru Forester rooftop, you gotta first have a universal multi-purpose roof rack base system. The Yakima Lowrider roof rack base system is designed for your 1999 Subaru Forester roof rack siderails. The Yakima Lowider system includes 4Lowrider Towers and a pair of crossbars. The Lowrider Towers mount directly to your siderails. You can then attach the cargo / luggage roof basket to the Yakima Lowrider front and rear crossbars. The 48" cross bars are the recommended size for your 1999 Subaru Forrester rooftop.
With the yakima Lowrider base rack system installed you can then connect the Yakima LoadWarrior cargo basket rack directly to the Lowrider base rack cross bars. This multi-purpose basket is ideal for carrying cargo gear and luggage. The Loadwarrior basket has all types of optional accessories to go with it too.
The great feature of the Yakima Load Warrior basket is that it has two integrated crossbars so you can then connect any of Yakima's gear carriers directly to these two integrated crossbars on the Loadwarrior basket. So for your needs you can attach any of Yakima's roof bike carriers and ski carriers. You might want to also visit
this list of Subaru rack photos or
this page of Subaru roof rack pictures to possibly see an example of this luggage basket rack system installed on a Subaru Forester with roof rack siderails.