Can anyone suggest how to make a large dining table for a covered porch?

I would prefer not to use a common picnic table style with gaps between the table-top planks, but I am concerned that the indirect exposure to the weather will be problematic.

Any advice as to material and construction technique?

It won't be rained on, but will be exposed to temperatures from -20 to 100 degrees F and humidity of 5 to 95%.

I'd like the table to comfortably fit 12 (and uncomfortably fit 14).

Thank you!