We tried to fit it in the lid to begin with.
Unlike Steve installing it vertically did not work for us. The element got too hot and cut out. We could get it going again but I think it probably would have ended up melting the lid.
We found that fitting it horizontally at the bottom of the tank worked best for our set up. We have 750L tanks and they are exposed to the elements being in the bed of a truck.
I will (eventually) get some pics posted if you want.