maybe if the styrofoam isn't working, you could put a block of foam on top, with at slit cut in it, and rest that (holding the mangrove) on the stryofoam. (the stryofoam floats, and the foam block provides support).
you can pick up polyurethane foam pretty cheap at craft/sewing stores. I've used it in quite a few tanks before without any problems.
Or maybe taping/gluing a disposable cup (with the bottom cut out) onto a styrofoam panel. It would still lean over, but it shoud stay upright.