Just go straight to Number of the Beast, Friday or Glory Road if you want to film a Heinlein book. Have Spacesuit Will Travel could be a big pre-teen hit.
Better going for Neal Asher's Gridlinked though.
There is an independant author called Peter Cawdron that has written some novels that would make good real SF films too. Xenophobia, Feedback and Mars Endeavour for example.