If you have a plastic guide, you can easily contour the back to match your larger shell. Then install it temporarily trapping it in place with the cables (don't bother adjusting the derailleurs yet). Jiggle it around to find the natural position where the cable routes are shortest, especially the FD cable. Trace the position with a crayon, remove the cables and glue the guide home with double sticky tape.

It won't go anyplace, since the cables will keep it in place, the adhesive is only to keep it from shifting around if/when the cables are slack.