Have you tried making one yourself? They are actually quite simple, especially if you plan on using the nib from the kit.
Simplest way is to drill out the primer end (primer removed!!) with a 7mm bit. Glue a piece of scrap wood about 3/8"-1/2" long on one end of the tube. Turn this down to fit inside the neck (bullet end). Rough up the mating inside mating surfaces of the casing and glue the tube in place. The tube is slightly longer than a .308 casing so a bit of trimming will be needed.
Viola! Your first casing ready to become a pen!!!!