I would pour baking soda directly into the bottom of the pail, as it may help absorb odors more effectively. I would also expect that a diaper pail will never be perfect. I have a Munchkin diaper pail, and it works pretty well unless you open it.
Other odor removers that can be effective include mild acids like vinegar and stronger bases such as lye. Both will actually chemically modify the odor causing chemicals, and both should be strong enough to get into the plastic some. Both are, however, fairly expensive solutions - vinegar in particular you need a lot of at full strength, and you need to really let it soak to do a good job. Lye can probably be used in smaller quantities, but still isn't super cheap.
You might consider a non-plastic diaper pail/trash can. Plastic inevitably picks up odors, and keeps it. We also have a metal trash can with a lid, and that seems to behave better than the plastic ones as long as we keep the lid on. It has to be 100% metal, or something will stick odor-wise.