I wouldnt use a bat or something like that. Beating a dog in the middle of a fight can actually make the fight worse depending on the dog. Basically I see it as if the dog doesn't know what is hitting them, they just know something is and as a result they are going to fight all the harder to try to get it off them--or worse you can turn a dog attack away from your dog to you.
Air horns are a great alternative. Stick it up by the fighting dogs heads---yes I know its going to be hard on their ears, but it will almost always distract them and with any luck it will get the dogs owners attention.
Pepper spray (Bear spray) is a better alternative than mace if you ask me, primarily becuase you can flush your pets eyes directly and give them some treatment if there is an accidental exposure. Also, there is less risk of allergic reaction with pepperspray than with mace. If you live in a place like NY, a "DIY" alternative is a ziplock baggie full of red/cheyenne pepper. Poor ti over the dogs face if it grabs one of yours. Just beware, pepper is going everywhere. Vinager also works, but is bulkier.
Just be aware of using mace/pepper spray you may expose yourself. Bear spray is not fun stuff.