DETROIT—Free to produce generic versions of the product after DoorBlocker’s 20-year patent ran out last month, hundreds of businesses have begun distributing cheap, off-brand doorstops to retail outlets nationwide, industry analysts confirmed Tuesday. “Now that anyone can manufacture a doorstop without fear of legal challenges from the DoorBlocker company, we can expect the market to be flooded with tons of these low-cost knockoffs of the original door-impeding device,” said market researcher Paul Holbloom, naming brands like DoorHalt, SwingLock, PropStop, and Wedge as being among those that have gone on sale in just the past few weeks.