Go to your local hobby store, or some hardware stores will carry it as well, and get some good CA glue such as Hot stuff or Bob Smith. Get the very thin stuff made for small gaps. Sand off the old glue residue and clamp it tight to check for a flush fit. If it fits nice and smooth clamp it tight and run the thin CA glue along all sides of the fix, you'll see it wick in. When it stops wicking in that's enough. Make sure you don't get any on your clamp or it will be stuck and hard to remove. Sand to finish and put a little spray on it to seal the fix and sanding marks.