Nothing to do with sublimation but thought this may be of interest.
I bought a pack of these the other day out of curiosity. Just a piece of cardboard but the result is very rigid, very good corners and quick to produce. Several sizes available but I hope larger ones will come along later. Price for 12" square is less than £5 inc canvas & ink , a backing board is available but at £1.80 each it is far cheaper to cut from mountboard. Cut size is 406mm square so this can be produced by a 17" printer.
https://www.my-accessories.co.uk/catshow/Artwrap___Canvas/Photo_Canvas.html