Hmm. For that price, if the core isn't huge and you need 70 mm, it'd be tempting to cut them down with a Dremel to fit your tanks. A little attention with the grinding wheel, and some sandpaper, will ensure the cut end of the spiral doesn't scratch the film (not a huge issue anyway, since you'd load from center out like any other stainless reel).

And if you hadn't thought of that, I accept Paypal as long as it's from a checking account or Paypal balance...