I just ordered a 16” sphere from Barnards to go with my LED analyzer project (Any interest in a LED/Battery analyzer device?) The price is quite reasonable (go up a couple of inches and it’s no quite so). http://www.barnardltd.com/product.jsp?prodId=1878&catId=950 No way you could justify making your own sphere at that price.
If it works out, I’ll probably get a 24” one.