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Hello, I think your mounting idea will be fine. You could probably also go with your midships idea as the EV-1 is designed to reject most types of interference. That being said, the construction of the boat could amplify the EM waves created by the radar and radio wiring so your idea of an external mount is probably the more ideal installation. Do make sure to separate the wiring from the EV-1 as much as you can from radar and radio antenna wiring and if they have to venture close to one another, a little grounded foil shielding would be advisable. And, per the instructions, make sure that the unit is level when the boat is at rest.
As an extra measure of interference blocking you just need to wrap the wire in some type of foil and, if possible, ground the foil. You can use professional shielding foil (https://www.amazon.com/foil-shielding/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Afoil%20shielding) or simply use aluminum foil. If there is a bonding wire nearby you can splice a small wire to it and have it contact the foil. That reduces EM interference to it's lowest point.