This is just a quick recommendation really....
About a year ago, we were in Skipton (Yorkshire, UK) and as I made a mad dash around a really nice art shop there, one of the things that got crammed into my basket was a 30 cm square 'natural linen' canvas board - made my the company Pebeo.
Well - a year on, and it's made it up on to the easel and a portrait is coming together rather nicely on it!
As far as I can see, they only come in a square format (but please do let me know if that's NOT the case) - and the surface is lovely. Transparent gesso, I think. It really takes the paint well.