Help with wooden Jambalaya pot lid

that sounds like a solution to a problem that doesn’t exist.
make sure you use a SS hinge and even then it’s going to corrode.

gonna be a bitch to clean too

I’d use a hard grain wood like sugar maple

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The main job of the lid , as simple as it sounds, is to seal tight and let the rice steam. If steam escapes, or just swells into the wood, then it will make the jambalaya cook inconsistently.
I know nothing about woodworking, but I know a lot about big jambalaya pots.

This is the plan. It'll be basically an upside down cutting board, with the juice groove being cut a little differently, and the groove will accept the lip of the pot.

My main fear is warping. I don't have much concern about a "tight enough fit". I'm guessing this thing will need to be about an inch thick.

I’d use a hardwood like hard maple, cherry, or walnut.

Use 7/8 thickness, Titebond 3 glue, finishes..if it’s going to be any of those hardwoods I wouldn’t stain or poly them. A good beeswax and mineral oil mix applied regularly would be good enough. But, if you go with a soft wood and stain it with poly, it needs a good while to cure to be food safe.

What about a board for a hibachi grill? Will use it as a cover when not cooking on it.

I didn’t ask. Maybe the lid that came with the pot is damaged, but I don’t know.

It would be used to cover only? Or food prep as well?

If it’s going to be just a cover for a storage situation, then use whatever you can find at the best price at 1” thickness.

If it’s to hold food on top of, then you’d want hardwood without any cracks or knots to limit bacterial growth. Or a softwood like cypress and seal it with poly and let it cure. Glue the joints where the boards meet to make it seamless. Use a titebond 3 or any other water proof wood glue

Your biggest concern will be warping and expansion due to steam from the cooking process, especially if making your groove to have the lid sort of seal the pot.

Walnut, purpleheart, yellowheart, and ash.