I know it is sometime difficult for some designer to hollow their models.
As I am designing many dice these times, I decided to allow dice designers to use templates of the 5 platonic solids (that is the regular D4, D6, D8, D12 and D20).
They are already hollow with a thickness of 1.5mm.
1.5 mm is suitable for Strong and Flexible material, for Detail materail and Alumide (and more, if you are lucky).
A small tunnel (0.01x0.01mm) is joining the external shell and the internal shell together (so that the internal shell will not be removed by the automatic check tools of Shapeways). The support material will get trapped inside (so your die will be heavy) but Shapeways won't charge you for that.
You can download them, choose the one you want and then decorate it as you wish (add numbers, symbols or whatever).