Yes, one kink in making that box as a DIY product is that much of the internal volume, like an iceberg, is below the surface - in this case, the surface of the load floor. It's a good bit of work to do.

We have made molds for some cars, but because of the convoluted lower portion of the X5, it would be pretty problematic to make one that worked for the X5 (not that I haven't considered it... : )

Here is a DLS Magnesium Series 10:http://kward1.homestead.com/X5subbox.html

You can see that it extends below that line...