Joah,
The furnace filter and/or pot scrubber approach is good for this. It makes for a much denser tree, and once the ground foam is on, they are not light and airy - they are positively "thick"
Another source for "armatures" is old (or new

) artificial Christmas trees. Their little branches can look quite good once they have been doctored with scissors and covered with ground foam. Although they are more expensive, I have also used Dollar Store conifers dipped in paint and covered with ground foam (they are usually 3 or 4 small, or 2-3 medium or 1-2 large for $1).
Andrew