Measure the length and width of your sofa and decide how much of the platform you want sticking out. The get the car warmed up and pop down to b&Q and buy some 4"x2" / 5x2" / 6"x2" planed timber ( ill hazard a guess that youll need 6 lengths in total just to be sure.
Also whilst your down there grab a box of 3" or 4" nails.
Take the overall length of the platform that you previously measured and cut two of your lengths of timer to this size.(1)
Then take the width you measured earlier , subtract 4" from this measurement and cut those.(2)
Now place the longer timbers(1) on the ground and fix the shorter timbers(2) in between them at either end.
Measure the length inside the frame and cut the rest of the timbers(3)
With the left over timbers cut some noggins to go inbetween the timbers just to strengthen it up a bit(4)
Fix it all together using your nails by fixing into the corners!
Cover the top of it with sometounge and grooved ply boards , i think they do them in loft packs designed for the floor of your loft or alternatively you could you mdf.
Then you can carpet it or finish it how ever you want!
Heres a pic i did in ms paint which may help with the numbers from the description!
Now get off your lazy backside and build it lolol
Keep us posted matey
Jon