Alternate sink plug

Not an idea to set the world on fire but I'd find it useful all the same.
I cycle to work so I have a shower every morning and often shave in the changing rooms at work. The sinks at work don't have any sink plug though , so I have to keep hitting the tap button anytime I wanted to rinse my rasor, which is wasteful. As a partial solution I can use the cap from my shaving foam can to block the plug(i don't want to steal one from home or buy one for work). Suction from the water draning away keeps it in place. But it don't make a very good seal, so the water slowly drains away anyway. If the cap had a rubber base though I think it would probably form a perfect seal. So... thats my grand idea, stick a rubber base on the lid of a shaving foam can, and market it as ideal for traveling etc.
I'd buy it anyway....