Something I do that you might try is use sturdy playing cards to try getting under the corners of the back glass (using a sliding motion along the edge of the glass) once you’ve heated the back glass of the phone (either with the iOpener or with a hairdryer/heatgun). Once you get a tiny bit under the glass, you can insert more cards under the glass and the already inserted card, then slowly go around the phone and cut the adhesive holding the back on. The phone should be hot enough that you can just barely touch/hold it, in my experience - make sure the glass is very heated, as the adhesive on there is quite strong and can cause the glass to break if you put too much lifting pressure on the glass before getting through all the adhesive. If you do it right, you can even keep the original adhesive intact.