Argh...sorry all, I didn't receive any email notifications!!
That is in fact a BMW part. Even more interestingly enough, it's a standard issue on BMWs in Europe not in the US. As you can see it's a first aid kit box. But the kit is not available in the US. The empty box is.
Part number is 52.10.7.044.958. Should be about $16~$20 ish depending on the dealer you call.
I spent a good hour trying to find good mounting pts. Even fabricated a couple of brackets. In the end, the best solution is to drill two holes on the top of the box and fasten with two tapping screws into two existing holes under the seat. Steady as a rock. The box moves with the seat so it's always within reach. And it doesn't hinder operating the slider lever either.
The box can hold multiple CDs, a small first aid kit, flash light, etc.
I will post more pictures in the evening.