Absolute noob looking to make octagon with exact dimensions

I need a thin octagon model with the exact dimensions as shown by this image. How would I do this in, lets say, Blender?

If you want to know the purpose of this, I’m attempting to port a board game to Tabletop Simulator. Got all the assets scanned, aligned, and cropped!

http://a.pomf.se/akyjgu.png

http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Add_Mesh/Planes_from_Images

http://dreamsteep.com/tutorials/blender/195-adding-a-basic-texture-to-a-plane-in-blender.html

The first link is actually very helpful and is also something I found myself a while back.

Now I have the plane and extruded it to give it thickness. What do I do to make it into an octagon?

you use the image plane as a backdrop and model on top of it. i don’t use blender, but there should be like, a default cube you can use to model it.

I managed to create an octagon! I used the image from plane tool, used the knife tool along the edges of the octagon in the image, and then extruded the model to give it thickness.

Also managed to UV map it!