I’ve thought about using some of that dense green foam that florist stick flowers in. It might give you a nicer finish. But how would you shape it? My hand skills aren’t good enough to make the pattern for a pencil holder look good. I would have to use a CNC. In the foundry, pattern making is always the biggest hurdle.
Foundry is a great process for making LARGE number of pieces cheaply. For ‘one only’ projects CNC is usually better.
Btw, I like having that Southern Bentonite in there, because it makes the sand flow into the mold better. A little of it burns out when the hot metal hits it, so you have to add some as you use the sand over and over.