I second Jerry's suggestion to use tulle. A search of the archives should tell you more than you ever wanted to know about the stuff.
But here's a quick summary: Tulle (pronounced 'tool') is available at fabric shops. It's what bridal veils, crinoline petticoats, etc are made of. It comes in a variety of colors; white should work best for a baseball unless it's really dirty.
Half a yard should be plenty. Expect to pay between $1.50 and $2.50 a yard. If the price is more like .99 a yard, that's berry netting which is not strong enough for framing applications.
Cut a circle of tulle big enough to hold the baseball. Cut a quarter-sized opening in the mat. Stuff the extra tulle through the hole, spread it out and hot glue it to the back side of the mat.
The advantage of using tulle is that it does not stress the stitching of the baseball. You have to look very closely to see it once the ball is in place.
Good luck.
Kit