what about a plastic lawn chair? You know, the cheap kind you can get for a patio.

My grandfather used this when he fell before they received their shower chair and found it to be almost just as good.

Other tips: a non slip mat in the tub, for sure.

If all else fails, anything you can sit on in the shower so that its not so hard to stand up all that time. Even a plastic footstool might work if you can settle down that height.