Really freeforming with sprang
It is possible to really freeform with sprang, working in multiple directions and crossing sprang.... I have been experimenting with this, trying different frames.
One of the frames I have used for this is a square frame. At the sides I use a strip of fabric with a tube for a stick or knitting needle and Velcro tape to adjust the tension. 
After spranging some rows, I start weaving through the sprang after each row. I can do this at the upper part and the under part at the same time or do it only at the upper part or the under part.