I'd argue the commercial solutions are value for money, given the complexities. By I accept I am biased :-)by Camayoc - Nginx Mailing List - English
I urge caution using this approach to a data diode. The question you ask is a very important one: where can I find a working configuration? Do not get me wrong, it is possible to make such approaches work, I have seen them in my companies test lab. The question you have to consider is reliability and trust. How reliable does the solution need to be? My experiece has been making somethinby Camayoc - Nginx Mailing List - English