I am a huge fan of using plastic wrap as a filler for large targets. It is free, easy to acquire and will not damage arrows. An added benefit is that it is impervious to weather, but that is not a concern of yours if it will remain inside.
I made a target with 'snow fence' material that I bought from Lowes. I used this plastic in lieu of chicken wire. If it wears out, you can replace it.
For filler, I hit up a large grocery store or Sam's Club. They'll give you some used shrink wrap for free. Pack it into your frame and the target will never wear out.
If this is a permanent target that you'll keep for a long time, have you considered buying a grass mat target similar to what is used in FITA?