Washer fluid cap 51mm / 2"

Though designed for a 2006 Toyota Aygo, it may just fit your washer tank opening, as long as it has a 51mm / 2 inch diameter snap-fit rim.

Printing

I printed this in ABS, but most thermoplastics will probably work fine. Washer fluid contains water, alcohol and detergents. Most filament plastics can handle those well enough.

Assembly

You'll probably need to ream the holes in the hinge parts to 1.75mm. There's no tolerance in the drawing, since the result will vary depending on your printer, print settings, and choice of material. I didn't have a 1.75mm drill, so I used 1.8.

Assemble the cap so that the hinge fits together correctly. Test that cap closes with a nice friction fit. This also ensures that you put it together correctly, before you finish the hinge. I skipped this on an earlier prototype, and assembled the lid upside down. It's a stupid error to make.

Take a small piece of 1.75mm filament. Stick it through the holes in the hinge, cut it off tightly. To ensure it doesn't slide out, I recommend that you heat the end of a pin in a flame and push it slightly into the end of the inserted filament on both sides. This will cause the inserted filament to expand and lock in place in the hole. Alternatively you could dab the outer ends with a little solvent. However, I tried that using acetone, and I found that through capillary action it creeps into the entire hinge and easily ruins it. Thermal deformation using a hot pin is much more localised and controllable.

Close the assembly, put it on top of your washer tank opening, and press down firmly to snap it onto it.

Finally I took a permanent marker and drew an icon representing the washer fluid mechanism on it.

UPDATE: If you want a nicer icon, print Chiribou Alexandru's cap remix instead of my cap, It has the logo as an inset, and is how I'd have made it if I'd had taken the time to make this.