Have you tried leaning the seat further back?
This would put more of your body weight back against the seat lessening the effects of the load on your lower back.
Also, the height adjustment, like on many cars, rotates from the front of the seat i.e. the front remains at an almost constant height where as the rear (next to the vertical section) goes up and down which has the effect of altering the angle of your seat base in order to give you the height adjustment. At it's lowest setting, therefore, it's most likely to give you much needed lap support.
I have found that on it's lowest setting with the seat lent back a little more gives me a more comfortable seating position. You may, as I have, need to move the seat a little closer to the steering wheel to give you better 'reach' comfort though.
Hope this helps!