Since that opening is very seldom well sealed behind the tub, doing that would often cause a leak when the cup was removed or if it did not seal properly. A suction cup would not work because there is no way to create a vacuum. It would have to be something glued on, or otherwise secured to the tub.: Please I need help, My tub has a drain mounted on the wall under the spout to prevent it from overflowing. However , as an adult it doesnt cover me enough. I would like to sit and soak but Im prevented from doing so. I once saw what looked like a suction cup at Woolworths that would allow the water to go higher than intended. I cant remember the name or the manufacturer. Can anyone out there put me in touch or tell me how I can locate one. Thanks P. Oliver