Hatch

 

It was difficult deciding how to crop this image - I made the original with the entire valve on the left included including it's wheel (of which you see only half here), but in the end decided that a clean left edge was more important than the valve itself and it was the shadow of the valve that was important, with just enough of the actual structure to 'explain' the shadow.

Whether to include the base of the tank I'm less certain of - on the one hand, the image is cleaner without, on the other it provides framing for the tank. I'll probably try it both ways before making a final decision.