I used a piece of Teak to mount my transducer to. I put a couple of 1/4" threaded studs on it and secure the transducer with wing nuts. I have two boats set up like this and can switch from one to the other in no time.

Plastic would work as well... but I had the teak laying around.

For filling in the holes, try the marine-tex putty. that stuff works for everything!