"...your grandma is going to do far better with something she can sit on, and preferably not a portaloo 2 blocks down the street when she is in a hurry."

I don't know about toilet seats there, but many/most of them here are made from painted wood.

How about this suggestion:

Remove existing bathroom toilet seats and screw blocks to the underside (choose screw length carefully) that will coincide with the inside of a 20 L bucket or a wooden box lined with plastic bags.

Sue