Two of these might do the trick, it has a small flat bottom though, but if calibrated i’m guessing thats not a big problem?
Way to hard to work with. Also, you don’t want a specular reflective surface on your sphere. Buy one of these: http://www.smoothfoam.com/product/10073.html
They have changed them since the picture was taken. The current ones have a flat spot on the ends… easy to sand into a true sphere. The flat spot makes the sphere a bit more sensitive to changes in the angle that the light shines into the sphere.
And while you’re at it, you will need one of these: Any interest in a LED/Battery analyzer device?