"Official" binders have thin metal rods inside the spine, each rod passing down the fold between the centre pages of a magazine. Taking my cue from this, I use old lever arch files, with the mechanism removed and string stitched through the top and bottom of the spine and running up the inside of the spine. Each magazine is opened out at the centre, and slipped under the string.
Problem is that, being retired, I no longer have access to old lever arch files with broken mechanisms.
Andy