I fly a lot for my job, and occassionally enjoy looking at the Sky Mall magazines that are in the seat back pocket.
They have a lot of really interesting things for sale, and I've seen laptop stands / holders for use in a car.
I don't think they attach or screw into the floor, but it may be worth a look to see what they have.
They have a website - www.skymall.com.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".