I've seen a few approaches that work, I used plastic weld (epoxy) and a Dremel. I V'd the broken areas out and thoroughly prepped the areas to be fixed then filled the areas in. I even had pieces completely missing and was able to create new using it with makeshift dams to hold it while it cured. I've got 17,000 miles on it, so far so good.
@hunt2elk used some pretty cool stuff on his, we'll see if he chimes in, also another member actually melted ABS with some type of thinner (acetone if I remember correctly) to create his own repair material.