Dear KForrest-
Many years ago I made a "wedge in" shower seat to fit the tub, for a friend who was wobbly after a stroke.
It was a 2" x 8" board cut to fit the tub with two end pieces fitted to the angle of the tub.
It could be wedged into the tub very solid and became a seat level with the top of the tub.
She could sit on the tub edge and seat from outside of the tub, then lift her legs into the tub and slide to the center.
If you have a friend with basic woodworking skills, perhaps they can make one for you.
This requires a traditional metal/porcelain tub. It might require different engineering for a plastic tub.