This tool can generate a TSIG key value (base64) for you very easily. The key value complies with RFC2845.

Secret Key Transaction Authentication for DNS (TSIG) is a secure method of authenticating clients making dynamic DNS update requests. The client application and the DNS server share a secret key value which is used to sign (HMAC-MD5) each update request. You can think of the key name as a user ID, and the key value as a password.

You can create a key value using four different methods:
Random 128 bit value, random 256 bit value, random 512 bit value or from pass-phrase (MD5-hash).

The key value generation functionality of TSIG Key Generator is already built into Simple DNS Plus and Simple Failover from JH Software.