My first though - 2 feet of quality 1x2 wood and some strong waxed or tarred sailor's marlin and quality whipping close the the seat and head lugs and on either side of the dent and that bike would ride just fine for the next 10 years. (That's stuff I always have on hand as a one time sailor. There are probably a hundred other ways just as cheap and easy to accomplish the same.) Repeat and ride another 10. And if this is a city bike, you've saved an easy pound. No U-lock required. The lightest cable out there will do just fine.