One can make a simple stainless hook with a handle on it, for lifting the lid out, set near the head..I'm guessing that on the occasional visits by the opposite sex, they will never reach in and touch a lid with (SURPRISE!) poop underneath, moss or no moss. A lid does make a lot of sense because it is probably better at sealing out critters. I can see where there might be a "caking" problem if any part of the stirrer is directly in the line of fire. In the commercial units, the stir mechanism looks like it swings out of the way, shaped like a C. I guess the idea is to just aerate a bit, not try to macerate it.