Row Wing Mounting Blocks

For the Mill Creek (or probably any other), what type of wood is used for the Row Wing Mounting Blocks?  I can see the placement measurements in the Mill Creek 16.5 construction gallery, but I can't tell the thickness.  Can I use exterior plywood and paint it to match the interior or do I need to use something higher quality?  Is it two pieces of 1/2" or some other dimensions?

Thanks,

Scott

   Never mind.  I found that the blocks I needed were in another box.

Scott