Lemsteraak
Adventurer
You know, I was thinking, Thule xSporter is an excellent system but it is a little pricey and you may not need the versatility is offers. If I were doing this I would probably go a slightly different path.
What about something like this?
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This is an ancient Maggiolina on an old Can Back system. It offers a nice feature that you can keep equipment in the back protected and shouldn't be that much more than the Thule system. I sadly can't recommend Can Back, don't think they are around anymore, but there are a bunch of clones like Best Top that should be able to do a better job.
Here is a link to the article
http://www.4wdandsportutility.com/newproducts/0303_4wd_roof_top_tent/index.html
What about something like this?

This is an ancient Maggiolina on an old Can Back system. It offers a nice feature that you can keep equipment in the back protected and shouldn't be that much more than the Thule system. I sadly can't recommend Can Back, don't think they are around anymore, but there are a bunch of clones like Best Top that should be able to do a better job.
Here is a link to the article
http://www.4wdandsportutility.com/newproducts/0303_4wd_roof_top_tent/index.html