I have used the Superior Acrylics baseball holder and I highly recommend them if you can't make one yourself.
However, if by "22 baseball shadow box" you mean that there will be 22 baseballs in it, the bought brackets are 12 bucks each plus shipping. That would be 264 plus freight to you. You just might want to run that by your customer before you proceed.
United Mfrs Suppliers has a series of molded acrylic ball holders (football, soccer ball, golf ball, baseball) that may be used with any depth frame. The holder protrudes out from the acrylic glazing, which comes with the holder.
Really neat. Really cost effective.
See Picture Framing Magazine, January, 2006, upper left corner of page 78.