Gotcha & ta! I've never worked out how to do it speedily, so I'll get into paint.net with mine & have a go later - is there a special 'function' I should look for?
Select the rectangle tool either from the tools window (using F5 or via the window menu) or from the toolbar button.
Then on the toolbar one can choose between shape outline / filled shape / filled shape with outline, so select the second of those.
With the default colours of black/white one simply uses the right mouse button when drawing the rectangles - or reverse the foreground/background colours and use the left button.
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