As sharp blades rotating at a couple thousand RPM can cause serious damage to things, it might be a clever idea to implement some safety features. So I built this:

The level of the blades is above the screews ends, so things should hit the screws first. Apart from the foolish mounting it seems to work quite fine.

It also has switches to sense hitting on something, although they are not connected yet. Only problem now is, that on high grass the cover tilts towards the front and the screws get stuck (towards the end of the video). But I hope that a proper mounting will stop that (although I have no good idea yet on how to build one, since it must move forwards/backwards and to the sides. Maybe a visit at the local hardware store will tickle my inspiration)