Lazy how-to question for people who've created contrib or core layouts - what do you use to make the layout icon? Is there significance to the size of white space around the block regions?
I've not created contrib or core layouts but a few custom layouts. For the icon, I changed the appearance of an existing one like https://github.com/backdrop/backdrop/blob/1.x/core/layouts/boxton/boxton... using a graphics editor.
Regarding the white space around the block regions, I'm not sure. Personally, I'd try to visualize the regions not exactly but only approximately.