I don't have any of the "stiffeners." Is there anything else I could use?

An alternative to stiffners is to crochet around a core. It is similar to basket making, where material is wrapped around a more solid or larger cane. Use a cotton yarn, paracord, or even a double strand of yarn as the core. Crochet around this material so that it forms a horizontal strand wrapped by crochet stitches. Avoid pulling the core too tight, as that will result in a warped surface on the next row. Sugar stiffening is most effective on cotton fibers, but will lose its shape if it gets wet. It also requires blocking - forming the item to the desired shape before allowing it to dry.