I usually start out the fire pan with sprinkle of sand and then make first camp. The sand keeps the bottom of the fire pan from warping.
At the end of the burn, there are usually some good pieces of charcoal that can be reused. So this is gathered and reused along with the ash/sand mix as the start of the next fire. Each time the ashes get reused, burning down to a very fine powder (Which is great by the way for compost). I prefer to return nutrients to the soil instead of to a landfill, but that might just be a NW mindset.
Reusing the ashes also acts as a protective layer on your fire pan - hence no need to add more sand to the mix at the start of each burn.
You have to pack out your garbage anyhow, finely burnt ashes take up considerably less volume (and weight) than partially burnt wood.
As a plus, having a separate ash can also allows for hot coals which might normally burn through whatever you are using for a trash bag (likely in some other container like empty cooler, ammo can, action packer, or your designated watertight container). Sometimes if cold I will get a fire going in the morning while waiting for breakfast and loading of the boats.
If the remnants of the fire are not completely doused, they are instead put in a metal tin (externally cooled in the water even?) and then suffocated they make excellent charcoal ...
the tin would hold a seal (I think) expecially if the bucket is held upright. The 5gal bucket could have a slight bath of water in it to keep it from melting or the tin from warping as the warm ashes (and charcoal) cool ...