maybe 100 characters that are easy to remember like your email address plus 16 to 20 truly random alphanumeric characters.

The email address makes it specific to you so a general sweep of the block chain by a password cracker doesn't pick you up. The 16 to 20 truly random part gives you over 100 bits of entropy. Not ironclad but good.

You know you will forget the password unless you store it in a safe place. Why not just export a secret key generated by a good computer algorithm to paper?