Why not make your own wood filler using the dust fines from the very wood that you are working with. Sand some of your wood down, collect the dust, separate the wood dust from the sandpaper grit and use that. Just an idea. I collect the dust from my ROS and keep it on hand for this very reason. If you collect the dust in a jar, shake it to settle out the grit at the bottom of the jar, you can use the stuff on the top. It should be grit-free.

Robert N Pruden