Methods for Increasing Nipple Size Using Vacuum Suction

I’ve noticed a bunch of different vacuum gadgets designed to make nipples bigger. They look pretty straightforward but seem wildly overpriced for what they are. A standard breast pump might do double duty, offering both the suction power and necessary fittings. Does anyone have suggestions on how to rig up a cup that holds its vacuum once suction is applied, allowing the hose to be detached?