If you want DIY then look here.
Kinetic Vehicles
Builder of the Mother earth news max car. Gets 100+ mpg with IDI engine and uses off the shelf parts.
Its Diesel only no hybrid system. I talked to Jack McCornack designer and builder and he said for the cost and added weight it was not worth the money for the estimated mpg increase of a hybrid system. With a little more R&D it could easily be made into a more practical daily driver.