I think it's a decent idea, if it works and is durable and it costs £15 its an absolute no brainer when the average e-bike is £5k, and a good dropper is £200 and the main weakness is mud on the shaft.
I think there are plenty of willing customers for this, just recently I saw a 3d printed
charger holder being advertised for roughly the same price and people were all over it, I'm sure that a £1 screwfix cup hook could perform the same task at a fraction of the price but never the less people were happy to spend extra on it "just to have the proper item"

I do still think you could make the same from an old innertube, but it might not last as long and it might take an hour to make. lets be honest most of the people who own £5k e bike, an hour of their time is probably worth £30-40 so it's probably a more cost effective option to just buy one of these. or use a massive mudguard(but I realise they aren't fashionable

)