I would also have the same concerns about the stiffening agent. I think I would use a cross hatch pattern and a long stitch length and baste it to a piece of muslin. I think I'd make the muslin larger than the aida cloth & bring the back to the front to bind it. Treat it gently whe dying, then dip starch it with a 100% liquid starch. Lay it on the counter to dry patting the weave straight. After it's dry, remove the basting stitches & muslin & clean your counter!