There's a
local quarry that had a neat way of filling very small gravel. A stove pipe is placed in the bag while its is open but laying flat. Fill the pipe. Lift the bag, pull stove pipe out.
With compost i'd swap out the pipe for something bigger. Like a 5 gallon
bucket with bottom cut out.
The key would be that the bucket is sized for the bag. Once bucket is full you are done.
The bags looked like sand bags. A wire tie closed it off.