PyQGIS: draw layer/feature's bounding box

Question

I want to display a vector layer's bounding box in a print layout (or possibly a single feature's bounding box). Does QGIS have anything built-in to achieve this in the renderer/style? Or will I have to load a style/add a symbol layer with bounds($geometry)?

Answer 1

bounds($geometry) in a Geometry Generator symbol layer will draw a box round every feature in a layer:

Best I can do for a whole layer with multiple features is a new virtual layer of the extent of the layer:

This should be a lot quicker than my first stab of the bounds of the union: