Back when I was a meat fisherman, and living in town, I thought that the best place for fish remains was in the dumpster of a nearby vegetarian restaurant.  It stunk anyway, fish guts weren't going to make that much of a difference.

What do you guys do with fish carcasses after filleting? 

Simple... wait a little while after it's covered in flies and really stinks, then stick in your annoying neighbor's mailbox.

I just let the coons and maggots have them in the woods near by house. The carcass is gone after a day.