I'm glad that you found this heating method works (and improved upon it). I have only tried heating liners twice before: 1) hot rice, 2) oven. With hot rice, I found it difficult to get the rice all the way into the toe. With the oven method I thought I may have ruined my liners. When I took them out of the oven they were gooey gelatinous blobs. Getting these on my feet & into the shells was very difficult and likely to cause them to wrinkle.
I thought of other ways to heat the liners in the shells. That's when I came up with the boiling water method. I thought that maybe turkey bags were necessary but it sounds that you found that normal plastic bags work as well. Also, I'm more of a DIY guy and I find that if I am careful, I can do a better job than a "professional" (because it is my stuff and I am patient).
I'd like to hear from others that have tried this method & see what they think. If you try it once, I think you'll find that it is pretty easy - and easy to edit.