Why an octagon shape?
I make lots of hotpads (I sell them) and it would be easy to make one this shape. I make mine like you would make a pillowcase.
You make the top and lay it face down onto the back fabric. Sew a loop on first so it hangs down into the middle. Trim the back to match the top.
Then add the batting-I use 2 layers of traditional but use what ever- and then trim I like to put the pieced top into the middle between the back and the batting.
Then just sew around the edges starting at the bottom and leave an opening to turn it with.
Then turn it right side out and sew a finishing seam around the edge closing the opening in the process and quilt it down however you want.
It is a good idea to baste the bottom edge to the batting before you sew around the edge.
This is just my way of making them.