I mount mine on plain painting canvases that I buy from Michael's.They come in tons of sizes. I decide what size I am going to make and then either leave and extra 3" all the way around that doesn't get quilted or add strips at the end to make up the 3". Then, you stretch the block over the canvas and staple into the edges of the fabric (not the block) and into the back of the frame.
This video is very basic, but if you search you will find more.
Watson
Mounting quilt on canvas