Fix a windscreen washer cap on a vintage Volkswagen

Old VW Beetles use the air pressure in the spare tyre to power the windscreen washer, in an ingenious bit of old German engineering. You might think this would deflate the tyre, but as long as you keep the washer topped up, the air used is negligible. But I digress! The cap on mine broke, leaving me helpless in a world of splattered springtime bugs. I needed to make a seal for the now-defunct second hole in the washer tank's cap. The repair would need to be adhesive and form an airtight seal around the ragged broken plastic, and yet be stiff enough to hold up to an air pressure of 30 PSI without deforming and being pushed up and out of the hole, AND be waterproof and be okay with having windscreen detergent sloshing over it as the car travels. If ever there was a job for sugru, this was it - and it worked perfectly. Thanks, guys!