built mine from 2 x 2 x 10  ends

I bought one of these years ago at Home Depot for about $70. Each shelf holds about 350 lbs, but I put 5 balls (on ball cups) and have loved it.  If I ever retire from bowling (not likely) I can use it elsewhere.

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=202866061&storeId=10051&langId=-1&catalogId=10053&MERCH=REC-_-NavPLPHorizontal1-3-_-NA-_-202866061-_-N#product_description

I made one out of PVC.  Holds about 15 balls.  Never thought about using ball cups and putting them on shelves.  Buy a piece of PVC 3" or 4" and cut several ball cups out of it.

NOT .. This is going to sound pretty stupid .. but .. I bought two of the shelving you talk too about 10 years ago and NEVER did I think to use them for a ball rack. I have them in my garage! Great idea!