I made my own ammo counter image in Illustrator, I've uploaded it here for you if you would like to use the file. I recently modified it to be up to date since i originally made it before the games release and you will need to scale it to the size you wish to use. I might have to work on it and release a cleaner to scale version later.
And other than that it was just some Styrene and clear acrylic to cover it. I Know everyone wants blueprints for this but really the pep file is your best blueprint. i cut out and traced parts from it to cut my parts to size and modified as needed. I'm still contemplating making a mod kit for this.