I should have guessed that!
Anyhow, guess I would have done the same. You never know what an extra piece of metal does to the sound and since we are only dealing with half of the strings the sound might become unbalanced with washers or tubes. Even if it weren't audible, you'd know that and keep a keen ear on any metallic 'Klang' on those strings.
By the way, that pencil sharpener (using a socket was really clever!) type of thing won't make dowels that has the same grain direction. Dowels like that have an end grain face. For plugs with the same grain direction as the body or any other board you'd need a plug drill bit. They're not too expensive at Lidl but you can also use a piece of tube with the desired inner diameter. Filing some teeth will help but you don't need much cutting strength for a few plugs. Remember, you won't be cutting through the material, in your case a few millimetres will suffice.