well the device does not have to be all that precise
people are just jamming foil in between the rain and the joycon itself
wood is just as sturdy as 3d printed plastic and cardboard can be made to be almost as strong as wood
you are worried about hard materials such as paper clips bending the delicate pin well then paper/cardboard coated in heavy duty foil would be much more forgiving on the pins
i mean this is not exactly rocket science here we only need to ground one pin
people have made all kinds of much more complex things from paper/cardboard
even more complex than the labo projects even
Well hackers and makers certainly know better than engineers. Let us know how it goes when you try it.