I used a hole saw initially, then had to sand the fiberglass to obtain the correct size for the screw-in-access port. CAUTION!!!! be VERY careful when drilling with the hole saw. The power wires from the battery are routed through this area going to the electrical panel. Also, the expanding foam used as flotation in the C-18 was very sticky before it cured, (this is why the C-18 is such a strong hull), and it bonds the hull and the liner together. It's not easy to break the cut-out free. Once you obtain the correct size for the access port, now begins the task of digging out the foam. I used a screw driver and a small putty knife to chip away, especially until you find the two #10 wires from the battery. Chip out, vacuum the debris, chip some out, vacuum the debris. These are tough little boats and your laying in a not so comfortable position. It takes time to make about a square foot area all the way down the the bottom of the hull. You'll want plenty of room to work inside the cavity. It's a nasty messy job, but the installation will look nice and professional when your done.
The transducer can be mounted with silicone, caulking, wax from a toilet ring, epoxy, and probably other stuff. I always have a supply of WEST epoxy on hand, so thickened WEST epoxy is what I used being very careful when mixing to not introduce any air bubbles in the mixture. What ever method you use, the key is solid material for the transducer to shoot through. Air bubbles, even small ones, will kill it's performance.
Don