Quoted from jibmums:

Here's how I flattened mine once I had them out of the playfield. You'll need a heat gun this time, the same 1/4" socket driver, and a small cup of very cold water. Lay the insert face down on a clean, flat, heat-proof surface like a granite countertop. Keeping the heat gun 8" or so away, you want to heat the insert as if you were spray painting it. In other words, don't just hold the gun on it in a stationary position, because you'll surely deform or even melt it. Move the gun and make "passes" over the insert, just like it was a model car and you were spray painting the body.
You want the plastic heated to the point where it's warmed/softened enough that the pressure you're going to apply with the socket driver will force it down/flat, but not so warm that the vertical sides, usually the thinnest part of the insert, deform. That's why you want to go carefully with the heat until you develop your rhythm for how to heat the insert. After several passes, take your socket driver and gently but firmly press down in the center of the insert. When it's sufficiently heated, you'll see it flatten down from the pressure you apply with the socket driver. Move the driver around in the insert to flatten as much of the area as you can. Then position it in the middle and holding it firmly down, pour enough cold water on the insert to fill it and to create a small puddle around it. Keep pressing gently for a minute or so while the water cools and hardens the insert, then remove the driver and let the insert sit in the puddle for a couple minutes, then dry and move on to the next one.
This method worked very well for me, and it was the first time I'd ever tried it. I need to stress again though, make passes with the heat gun instead of holding it in one spot! While it doesn't make the inserts perfectly flat (only sanding would do that) it eliminates all/most of the banana shape. And if you do then sand them, you'll need to remove less material to flatten them entirely.