Flipper bats have quite a bit of cross-bracing molded inside of them, and sometimes that cross-bracing breaks from lots of use, making the flipper bats have a "spongy" feel. I've had to replace a few flipper bats due to this. I don't think you'll get a satisfactory feel, or longevity, from a hollow flipper bat. You'll probaby have to look at existing designs and come up with some kind of similar internal cross-bracing.