I puched mine out by parking (sliding) in to a snow bank one winter. I never put anything back in its place.
http://www.realoem.com/bmw/showparts...58&hg=51&fg=15 I think those pins are supposed to go all around. 2 top, 2 bottom, 2 on each side. The slats assembly is then held in place with the one-way washers you describe. It's part number 64111364257 (#10). There's a picture on the diagram, but it's not great. Google image search "internal toothed washer" for the general idea. I'd get the BMWs ones though, they call them "spring nuts". Should be about 25¢ each from BMW.
In addition, the sides have spacer pieces. 51112251389 and 51112251390. I'm guessing they go between the slats and spring nuts, but I'm not sure. Here's the two I have from my car after I punched it out.