Yeah the Thule 604 Ascent 1600 cargo box connects directly to your 2000 Subaru Outback factory rack cross bars and is the best option for your needs in my opinion. The size of the cargo box matches the Outback for a good looking match up. Ideal dimensions for your needs too. It has tons of room for gear for 4 people, including snowboards, camping equipment and general cargo gear.
This is a pretty cool tool for you to preview how this cargo box will look on the Subaru Outback too.
http://www.orsracksdirect.com/thule-cargo-roof-boxes.html
(click on the 'Thule roof box comparison tool and visual guide' link on the page)