You do not want to use an off-the-shelf MAC; you would prefer just to use a hash function.
Don't do it.
Many things can go wrong, and there's really no reason not to use one of the excellent existing solutions. Nonetheless, some people believe they can do message authentication in a straightforward manner using a hash function, and they believe they would be better off doing this than using an off-the-shelf solution. Basically, they think they can do something less complex and faster with just a hash function. Other people think that creating some sort of "encryption with redundancy" scheme is a good idea, even though many such schemes are known to be bad.
OMAC, HMAC, CMAC, and MAC127, which we compare in Recipe 6.4, are all simple and efficient, and there are proofs that those constructions are secure with some reasonable assumptions. Will that be the case for anything you put together manually?