Fill cupped inserts with 2 part auto clear.

Sand them before they fully harden.

Usually heat does not un-cup them for long, because without the reinforcement of the jeweling, they sag again.

Note that even NOS playfields often have cupped inserts, so you can't blame the lights for melting them...

Does that work?

How does it look?

Get a chem respirator ($25 at Home Depot), gloves, and even a Tyvek bunny suit ($9.99 at Home Depot) if you want to be really careful.

Do it outside if you really don't want to buy the respirator, but really you should wear one even when using rattle cans, so get one anyway.

Level the playfield. Use a glass eyedropper, and clean it with Lacquer thinner when you are done.

Here is PinballDude using 2 part auto clear to level inserts with no chem respirator (not recommended), but you can see how he does it.

He sprays it with just a dust mask too (really not recommended).

But doesn't it just sag again in a year or so?

I have never been able to simply mold them flat again and have them stay flat.

The auto clear reinforces them.

Depending on the speed of the hardener, maybe a few hours.

If you wait too long, it becomes much harder to sand.

Practice on a scrap of varnished wood with a dent from a ball peen hammer. You'll quickly get it.

Even New Old Stock playfields of old Williams and Bally games often have cupped inserts.

They have never been used, so ball wear did not cup them.

They cupped because the cheap plastic is thin.

Latter on, most inserts had "jeweling" (those plastic rays on the back of the inserts) that reinforced the insert face and kept it from cupping or sagging.