this one incorporates grains as well
I have seen maggot feeders for fish ponds or even as hung as fish attractors for wild fishing. Chickens need grain for calories even on pasture (grass, weeds, bugs) just less. Maggots would be a good protein supplement but, unless the road kill is really far gone, I usually just thoroughly boil the carcass until the meat is soft and feed it to them directly. Chickens are not picky eaters thoroughly boiled rough fish, where the bones are softened, will be eaten completely. I usually get a good egg increase too but the still need greens for vitamins.
brewery grains and animal manure instead
These are a different kind of fly, not a house fly, that I did a post on once.