OWA access thru internet

We have an exchange 2000 server named ex01 which are clusterd to form a virtual server exvs01, and two bridge head server ex03 and ex04 which routes mails, we have configured OWA on one of teh front end server Ex04, and able to get check mail thru internet explorer within office network, we want to publish this owa access through internet, we have avantail fire wall installed, How do i configure OWA access thru internet using this avantail box, without using microsoft ISA server. also what security need to be checked and implenented in order to be secured. kinldy let me know if any documents you have for the same.