Use a Shell181 which is a 4 nodes structural element or a Shell281 to add midside nodes. If you are working in Workbench, it will automatically use Shell281 unless you change the Element Order under Mesh Details to use Linear elements.
If you have a complex extrusion shape, you can use the Midsurface tool in SpaceClaim to pair up the faces and replace a solid model with sheet bodies that are connected using the Share button on the Workbench tab.
Regards,
Peter
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