This parametric shock cord mount will let you make a mount to fit any sized body tube. I normally secure my shock cords to the motor mount, but if you're retrofitting an existing rocket with a new cord, this might be just the thing you're looking for.

Note that many adhesives don't work well with printed plastics, so test your adhesive on a scrap piece of tube before trying it on your actual rocket. If the glue doesn't hold up to the ejection charge's force, you're going to have a bad time. Good layer adhesion is also important, give the loop a good tug to make sure it's going to hold up before you rely on it in flight!