or this, but it's only 15", https://www.amazon.com/Gecko-Loc-Shampoo-Conditioner-Organizer-Stainless/dp/B01I1NRMA8?ref_=ast_sto_dp&th=1&psc=1
I went to their storefronts on Amazon, then a google search for their websites. They both have some other options.
I did a google search for 'heavy duty suction cup shelf 24" ' and then clicked google's shopping tab and saw a bunch of hits for window plant shelves. Maybe you'd find something in that category that might work.