I started with a set of Quick and Easy roof rack brackets. Though I got a set from a member of http://www.bmw2002faq.com/, they are available online for around $80 for four.
I bought two pieces of 1-1/2" aluminum angle for the crossbars. For the slats, I (ahem) recycled a wood futon. A few carriage bolts and some careful drilling resulted in a useful and sort-of-vintage looking roof rack for about $100.