If the broken ones are on the end edges of the panel, which mine were, I ended up drilling into the metal, using a bottoming tap to start SOME threads, then used 3/4" long pieces of threaded rod (with JB weld) screwed into the panel and left to set. Might sound a little mickey-mouse, but it works.
You just have to be careful not to drill all the way thru the panel where the broken studs are.

My panel cinched up tight to the trunk lid and has been like that for 3 years now.