How do I make a custom IPSW for iPhone 3Gs in sn0wbreeze without jailbreak

I don't want a jailbreak so I can unlock properly (phone is locked to orange uk and i use three), and the process on redsn0w to downgrade baseband jailbreaks it again (which i don't want to risk), I have downgraded from 4.0 to 3.1.3 (don't worry i have backed up the blobs and backed up an ipsw for restoring) so i have the 05.13.04 baseband and I don't think that baseband will work on anything above 4.0.

tldr: i'm stupid and want ultrasn0w but don't want to break my device installing the baseband multiple times and also don't want jailbreak messing up.
 
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