Ah, I got here too late. In case anyone else is still having the problem. The cap from a tube of toothpaste usually replaces the Proraso tube cap just fine (a little force may be needed at first). Err, as for what to do with the toothpaste tube that is now lacking a cap....... well shaving is the priority here.
Razors don't shave people. People shave people!
There are three critical types of moments in life: Times when we should use opportunity to be enamoured with finer details, and times we should ignore the minutia.