Comp Fill
Fills a shape layer to the size of the comp, like a solid but self-contained, auto-sizing and easier to customise.
Tutorial, 0:16
Make a new shape layer and apply the effect: the layer fits the comp straight away. Change the comp size later and the layer resizes to match, so set it and forget it.
Install it under User Presets, then Shapes, and refresh the Effects and Presets panel.