Think your missing part of the point regards a seat/frame.
Yes it can be a pain in the backside dismantling etc however....
Its all about comfort and being able to concentrate without distraction that allows you to enjoy the racing games. On a good racing frame you could sit for 3 or more hours with no major aches, thats what you need to achieve.
If you arnt comfortable and eventually find your giving yourself a bad back this will put you off using just a table of sorts so you still wont benifit.
Look into ebay for proper bucket seats from as little as about £60 and consider your own d.i.y frame thingy that will allow easy dismantling and using the seat as a chair on its own perhaps?