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I started trying to do "floating" mounts a couple years ago, simply because I wanted to be able to periodically replace the print, and reuse both the mat and frame. My method was like yours. I would tape the corner edges of the print to the back mat and hinge on a window mat.
No matter how tight I tried to "stretch" the print, noticable "waves" could be seen in the finished product. Even with very flat sheet prints. So I tried a 3M "postit" double sided tape that I had used years ago to hold paper flat on a darkroom easel.
The tape works great. However, my oldest mounts with the tape are only about 1.5 years old, so time will tell if they continue to hold. My biggest was 16 X 20.
The 3M tapes come in a variety of forms and sizes. Just look (or Google) for "double sided" and "removable". The tack is strong enough to old things flat, but easy to remove. Once removed, any residue left behind comes off easily with a little light rubbing.
My method is to put down 3 or 4 large "X" patterns with the tape, lay down and position the print, then press lightly to secure. I still use corner mounts. A small roll of the tape is only $3 to $5, so it's cheap to experiment with.
Here's one source: http://www.amazon.com/3M-Removable-Double-Sided-Tape/dp/B001390CK0