I looked at the ingredient list of the spray bottle of shower cleaner I had purchased and found that the main ingredient was citric acid.
Well, I have a jar of it on the shelf from a wine making kit. So, I added one teaspoon of the powder to 1 cup of water and a dribble of dishwasher rinse agent. This went into an empty spray bottle and I put it in the shower.
The stall gets a spray down every day after my shower and it stays looking good for much longer at the cost of only a few pennies and some effort.