MX-5 2016 has very little space to store things. There is a cubby compartment between the seats, which is tall enough to be divided by a shelf. I found inside the cubby small ledges on each side and 3 slots on the back wall, which can be used for mounting a shelf. Measuring inside that space is not easy; I did some preliminary measurements and cut out from cardboard a shape of this shelf. After a few iterations of cutting and fitting, I made a pretty accurate model in Solidworks and produced a DXF file for laser cut.
I ordered from Ponoko a laser cutout from a 2mm acrylic sheet (slots will only hold 2mm tabs). Fits well, but very flimsy. So I decided to sandwich two plates together (breaking the tabs on the second plate) and covered it with felt. This turned out to be a nice addition to my car.
I shared my design file and a few other people made their own version of this shelf. Good blog about it here.
updated on 2/16/2017
Shelves available for ordering now are made from black 2.3mm Derlin plastic. I found it is a much better option , even though it is more expensive than acrylic. It is more flexible and the supporting tabs are almost unbreakable.