You can basically just feed the various inputs in to a hash function. Of course, this imposes an upper bound on the amount of entropy if the output in the form

You can basically just feed the various inputs in to a hash function. Of course, this imposes an upper bound on the amount of entropy if the output in the form of the length of the hash function output, but this is rarely an issue in practice. Edit: Of course, this introduces the assumption that your hash function is secure, but again, not a problem in practice.