Really freeforming with sprang
It is possible to really freeform with sprang, working in multiple directions and crossing sprang.... I am still experimenting with this, trying different frames.
One of the frames I use now 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.

crossing freeform sprang

After the sprang in one direction is finished, I can manipulate the weaving part, as I did with this sprang.

freeform crossing sprang