I need to buy some plastic for a project, and need advice on the best for my needs.

These are side guards for an equipment seat.
It needs to be.........

12 inches by 8 inches.
1/4 inch thick.
Black in color.
STIFF and as inflexible as possible.
Won't crack after drilling holes and bolting mounting tabs on it.

The tabs will bolt to the bottom of the plate and up at least 4 inches.
The tabs fit in a slot with the plates sticking straight up.

This will be subject to side loading against the plates.

What plastic?? Delrin??