I have a couple of fittings that are the same thread as the nipples on the fuel rail and a little steel ball that goes in it to block off fittings. I got them with my dual CP3 setup so I could take the second CP3 off line in the event that it had a problem. I think you should be able to source some fittings somewhere and a check ball as well.
Ive got quite a few old injector lines here Josh - wont be a problem blocking them off - thanks