At least for wood charcoal an external heated retort is more efficient than a simple drum with an smothered fire in it. For an example see:
I do not know if agricultural waste has enough volatile combustible content to work in such a charcoal retort it may worth a try.
Another consideration worth may be another form of briquette. A round hollow briquette that snugly fits into the lower part of the rocket elbow and burns there. See:
Another form of briquette that seems to be mainly used in Asia is the honeycomb briquette. While it may burn in in the say way as the briquette above it may be more complicated to light.
In Asia the honeycomb briquette is often made with clay as a binder. This briquettes leave a fairly hard clinker that will not fall through an ash grate. If this briquettes should be burned in an rocket the elbow has to be modified to an tee. The second opening of this tee can be used to remove this clinker.
Some links to stoves that use the honeycomb briquette