How to avoid warping on flat FUD parts?

As you can see, this part is quite heavily warped. Of the six I ordered, some are almost flat while others look like this or even worse.

The flat part is about 0.5mm thick with an 1.5mm frame around it (you can see the rectangle frame in the image).

How do I avoid warping? I considered the following options:
[list type=circle]
[*] Make the part thicker. This will make the model less realistic.
[*] Fill the area inside the frame. Not an option as I will add a metallic weight there.
[*] Add a support beam in the middle. This, again, will make the model less realistic.
[*] Print the part sideways. Not really an option as one side would then suffer from surface damage caused by support wax.
[/list]

What about a removable support beam? Something that could easily be snipped off, that runs the length of the item. Another option is warming it up in some warm water, and try to flatten warped parts.

You can rescue the model by pouring some hot water over it.

Thanks for the hot water tip!

Will this fix the part permanently?

It should do.

This would happen to the prototype too, if it were not for the underframe.
Even injection molded items have a representation of an underframe, to keep the model square and rigid.