It's highly likely that the Mandalorian mock-up was the inspiration for Jango Fett’s final look.
In summer 1998, Cinescape released an all Episode 1 issue (Cinescape Insider #4). The issued rehashed the same information which had been discussed in its parent magazine. The Mandalorian cover photo appeared in a much smaller form on page 19 within the issue.
If you watch the documentary "The Beginning: Making Star Wars Episode I", you can see a portion of the cover for the Episode #1 issue. A clipping of the cover’s headline “Star Wars: Episode One Secrets Revealed!” is posted on a door in ILM. (See below for photo evidence. A link to the segment of the documentary is also below. Tthe cutout appears at the 1 minute 40 second mark).
So, we have evidence that Cinescape Mandalorian photograph was in ILM during the summer of 1998.
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[flash width=425 height=344]http://www.youtube.com/v/nBjf3I_2fOw[/flash]
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Originally, Jango Fett’s outfit was to be all-white, like the original design for Boba Fett. However, Lucas decided that he liked the unpolished armour better. So, when Doug Chiang's set out to finalize the stainless steel look for Jango, he either consciously or unconsciously drew upon the Cinescape mock-up for inspiration.
Hear are a couple more preview videos that have not yet been posted:
The Mandalore Plot Episode 2.12 Preview
Jon Favreau joins the Mandalorian Legacy
Here is a link to a short article about Lucas’ vision for the Mandalorians in the TV show:
Starwars.com | Mysterious Mandalorians Revealed!