I printed some of Max fixtures and altered them a little so they can fit the mounting holes on the IKEA system. In the end there will likely be a ton of different fixture combos that cover all kinds of situations.

I think the picture speak for themself. The fixtures are really nice you can snap the lower part onto the rails and then slide in the top part from the side.
The prints are not of the best quality but they work perfectly mechanical wise.
I will add some photos later when I actually mounted the whole thing on the IKEA beds.

This one uses a slightly modified top part, instead of the hole for the nut it has a 11mm hole that fits the little mounting hole thing on the ikea hydropnic beds ( compact: http://www.ikea.com/de/de/catalog/products/90319065/ bigger one: http://www.ikea.com/de/de/catalog/products/00319003/). If you zoom into the compact light fixture I linked you can see in the top right corner the mounting hole I'm talking about.

This is the normal middle one. I didn't use countersunk screws as I didnt have any but normally you would use a countersunk one.