I measured a new one that I have in my spare parts box.
The internal diameter of the cap is 3.61 mm (.142"). The nearest fractional equivalent for the ID is 9/64".
The outer diameter of the cap is 7.5 mm (.2955").
The wall thickness is 1.23 mm (.0485").
The overall length of the cap is 11.98 mm (.472").
I made the measurements with a caliper. Since it's rubber and may not be perfectly round, the measurements may be a little off from what the manufacturer says the measurements are, but they're going to be really close.
Yamaha's part number for that cap is 895-14169-00-00. It's just called a "cap". They're sold on Amazon for about $4.79.