Antifreeze in the base? I'm not sure what your instructions involve, but I know that with the humminbird you can't really get good "dry" results because even the slightest bit of air between the face of the transducer and the hull will throw it off.
To test position, they recommend putting the transducer on a ziploc full of water, sitting on the hull = no air beneath.
See this, that I just copied & pasted:
How to Install Shoot-Thru (Epoxy In Hull) Transducer:
First: It is recommended to test the location prior to installation. Hull construction can have effects on getting good readings and it's best to find the best location. Air Pockets, Areas that Glass Matts Overlap, Delamination of Hull can effect the sonar performance. The best wasy is to disable automatic bilge pump temporarily and add a couple inches of water to the hull. Next build a small sand bag using a Ziploc bag. Place transducer in hull and using sand bag hold it down to find the best location for installation. Normally the closer to the rear of the boat and center of the boat the higher speed the sonar will stay locked on. Make sure the location is away from strakes underneath as these will cause aerated or turbulated water underneath and disrupt the clean water flow accross the bottom of the hull which can effect sonar readings.
Second: Mark the location of the best readings and drain the water and allow hull to dry. Hull must be dry for epoxy to adhere and set-up.
Third: I like to sand or smooth out area of hull the transducer is mounted so it's flat and smooth surface.
Fourth: Using Plumber's Putty or Play Do build a small dam around the Area the transducer will installed about an 1" wider than transducer to keep epoxy from flowing during installation.
Fifth: Use Slow Cure Epoxy - I recommend the epoxy that comes with the transducer direct from Humminbird. Slow Cure Epoxy stays cooler than fast cure. DO NOT use silicone or any other material than epoxy. Silicone can absorb sonar waves and creates alot of issues. Mix thoroughly until consist color. Avoid whipping while mixing to reduce creating air bubbles in the epoxy. The less air bubbles the better the final results and improved sonar readings.
Sixth: I like to apply a thin layer of epoxy on the transducer, I also like to apply a thin layer pressed into the bottom of the dam to squeeze out any potential air bubbles between epoxy and hull. Fill the rest of the dam with remaining epoxy.
Seventh: Installation of transducer. As you push transducer into the epoxy start 90Ã‚Â° from foward and push and slowly turn the quarter turn. This will helps squeeze out air bubbles under the transducer.
Eighth: Using Sandbag from step one place on top of transducer to hold it in place. And allow the epoxy to cure for a minimum of 24 hours.
Ninth: Remove Sandbag and Plumbers Putty. Your Transducer is now installed and ready to use.
Tenth: Test Transducer in the Water to make sure you have good sonar readings. If not you may have air the epoxy and may need to repeat procedure to get good sonar readings.