Have your ever considered standing while sewing? I do. Someone told me she stands because her back hurts. I don't have any back problems, but if I sit too long, my feet swell. I put bed risers under my sewing table and I stand while sewing.
I have this on my computer chair and it definitely makes me taller in the chair.
http://www.isokineticsinc.com/mm5/me...Category_Code=