Cargo net across the passenger seat??
Check out a sporting goods store for a bag made specifically for bats/ baseball equipment. Should be able to find a smallish one to throw on like a backpack. That way it won't move around and it;ll sit lower.
If you have the typical long exhaust up the side of your bike, you could always strap the bat to that. Give it some stylish burn marks- or wrap it in foil.