I have been thinking about the best way to mount my canvases; on stretcher bars, there should be an ⅛th inch thick foam board for backing. The thick canvas I use now is not stretchable. Don’t let anyone try to stretch it; it would ruin the canvas.
Without the sketcher bars you should use a ¾” or ⅞” foam board. Make sure that the surface and the core are made of archival materials.
Do not staple the canvas on the side of the backing. Only on the back side. Or not at all.
Do not glue the canvas to the foam board, unless you use something that is removable. I do make the canvas water proof with acrylic, so it won’t expand much.
⅞” or ¾” plus the canvas should fit into a frame for a 1”. Like these: