I haven't tried it, but this https://bikegang.ecwid.com/?3vOM2tpoyAUSFIWU-Te2WMunXOCIAUaWhSyxRHLzwBUfmyTqrZ6dOUI9E#!/Brompton-2-to-3-Speed-Sprocket-Cog-Thumb-Shifter-Kit-MiniMODs-x-Sturmey-Archer/p/115439619/category=9803294 looks like a cheap and simple way to try this mod
Most of those parts are available individually on AliExpress and similar.
Usually for less money.
For ref.
@dmcg started
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I haven't tried it, but this https://bikegang.ecwid.com/?3vOM2tpoyAUSFIWU-Te2WMunXOCIAUaWhSyxRHLzwBUfmyTqrZ6dOUI9E#!/Brompton-2-to-3-Speed-Sprocket-Cog-Thumb-Shifter-Kit-MiniMODs-x-Sturmey-Archer/p/115439619/category=9803294 looks like a cheap and simple way to try this mod