Enforce weak RSA in TLS
Does anyone know some good way to create such a certificate?


As you need to put the private key on the server anyways, you could just copy it of and use that for demonstration purposes.

Alternatively you could use software versions from the time when such keylength restrictions were not in place.

In particular you might want to take a look at FREAK, in particular this website (need to scroll down a bit) has a list of vulnerable server and client software that you should be able to retrieve from release archives for demonstration purposes.