Sawdust, peat moss, and top soil would all help.

The most effective IMO is dry charcoal from a firepit. Some ash mixed in is okay, but heavy on the charcoal component.

Most of the smell actually comes from the "liquids" component as bacteria break it down. Separate buckets is a good idea.

Charcoal works so well that you can create a separate "liquids" bucket in the house and there will be basically no smell at all. Fill 3/4 with charcoal. Sprinkle a little more on top after use; or squirt top with soap/water mix in a spray bottle. After a lot of use, the charcoal will start to float -- and still no odour. I did this during bathroom renos and believe me it works.