The Helix 7 should be the largest that you will be able to flush mount. Depending on what youre wanting out of your setup, the Helix 5s and 7s are not ethernet capable, so you wont be able to share waypoints and other data between them. For the money, the 9s seem to be a great product. If you arent dead set on flush mounting, you could always get a mount from Sherms or Precision Sonar. I have a 999SI in my console now, but I will be going to one of those mounts when I upgrade next winter.
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