If I have something that is close in size at home already, not much, maybe a buck plus shipping. If I actually have to buy stock, probably just what the minimum cost it would be to get the stock and the shipping. Lemme check and see what I got at home. If I can find a scrap left over that is big enough, you can just pay the shipping.
Also if you have calipers you may want to take an exact measurement of the opening. Use the smallest measurement you can find in the whole diameter. As little as 2 or 3 thousandths of an inch could make the difference between it fitting or not if you are going for precision. If there is a shelf that this is going to sit on, it could be a little less critical since we could slightly undersize and still be close.
I'll let you know what I got at home or if I will need to order materials.