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A bit more searching found this:
https://www.amazon.co.uk/designacable-3-5mm-Stereo-Mini-Jack/dp/B00TS9T596?th=1Or you could buy a stereo to mono 3.5mm to 1/4" adapter instead of the one you're already using?
https://www.amazon.co.uk/Hosa-Adaptor-3-5-MM-TRS/dp/B000068O3UI'm not recommending specific models here - these are just the first examples I've found that appear to tick all the boxes.
https://www.thatcable.com/3m-pro-3-5mm-stereo-to-6-35mm-mono-jack-plug-cable-1-4-adapter-guitar-amp-lead
Probably too long - do you solder?