That is going to be pretty damn tough to do without the source code to the OS. What you need to understand is that these days, any worthy scanner uses specific characteristics of the IP implentation of the OS to make a guess at what it is.

This includes things like how the sequence numbers are generated, windows, how it responds to a packet with certain flags, or all flags set. etc.

Just changing the banner hasn't cut it since the late 90's.

Sorry. Maybe you should just go ahead and run Linux or a BSD. That way your box will be sure to show up as a Unix flavor during port scans.

-- spurious