I mentioned it in the previous* thread in this community, but a similar thing to this that someone could DIY is retro-direct, which uses 2 freewheels and a longer chain (jockey wheel OR double chainring). This has no switching though, instead pedalling backwards is the other gear.

measure to not use a front-derailleur

Not really specific to front, especially when that’s more of a thing for additional gear range.

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I saw Berm Peak made a fundraiser again with WBR, so I can’t help but repost this article