I use a Pilot Birdy mini pen, it runs around $7 on Amazon and there's a mechanical pencil version as well. It's skinnier than the pen loop, but fits in nicely and the clip on it fits right over the pen loop so it doesn't fall out super easily. That said, if/when it runs out of ink/is lost/is snagged by small nephew and not returned, it's pretty easy and inexpensive to replace or have a backup laying around.