Re: Aspect ratio frustration
That's an interesting idea - hadn't thought about metric. A quick google doesn't show signs of anyone in the US carrying a 40x60cm, but Jackson's will ship and the shipping to the US is not impossibly high.
I looked at blick's supplies for stretch your own and it's not cheap but I'm more worried about badly stretched canvas.
Painting on a border / mat is an approach I'd have to think about. Seems like it would be distracting. But it would be cool to have a trompe l'oeil mat that looked like and old carved and gilded frame....