A while ago, I saw a guy clamping an old plastic bottle between the seat stays of his bike, to use it as a fender. It doesn't look very stable but it was a great idea anyway!
This 3d printed mount works with most plastic bottles, like Coke, Evian ect. You can just clip it on your bike - no glue or screws needed!
I've used a bottle which I've cut before. Technical, there is no need to halve it but it looks better this way and it's done easily.
This bottle-fender-mount works nice on old steel bike racing frames. The diameter of the tube should be about 29 mm (1.14 inch). Please check the pictures above to see if it's comparable to your bike.
if you need it for another frame size.