The round hole cover is model number 16 in the game. It is located in the sewer of Tonobio Town and needs to be obtained after unlocking the treasure chest in the pool. Let me introduce to you the specific location of the model.
Enter the sewer first
Hit the button on the wall to turn on the light
Go all the way down the stairs
You can see a treasure chest at the bottom of the pool.
Climb down with the ladder
Defeat the rat below to unlock the magic circle
Use the magic circle to turn the valve to drain the water from the pool
Open the underwater treasure chest to get the model round hole cover
