What do you do when you upgrade your electronics specifically the transducer? How do you seal the mounting holes of the old transducer on your fiberglass boat?
I thought of just sealing the holes with marine silicone, is this enough?
I have also thought about mounting the new tansducer and speed wheel on a piece of plastic and then mounting the plastic to the transom. this way if I ever want to change things I can just drill new holes in the plastic and not the boat. Any other responses or ideas would be appreciated.

Boaters World in Everet,sells the plastic your looking for. It comes in a bunch of different sizes,and is about 1/2" thick. As far as sealing the holes,look into 3M 5200 marine adhesive. It's what I used when I mounted my transducer. I shot some in the holes,and also coated the screws with it before mounting the transducer bracket.I think I'd just remove the old screws,coat them with 5200 and put em' right back in the holes.

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!

silicone works on mine. Silcone,marine-tex, and duct tape kept my boat afloat for a year

Marine Tex is the way to go. If it is a through hole you are trying to fill I apply marine tex from the inside of the transom. Let it dry and then fill the rest of the hole with 5200 from the outside. When the 5200 is dry I cap the hole with some more marine tex. If you even want to get carried away; sand down the marine tex and apply gelcoat to match the color of your boat. If done right you can't even tell where the hole was. Anytime you put a screw into a glass boat make sure it has a bead of 5200 on it. The stuff is permanent and is easy to work with.

I recently replaced the outboard bracket on my boat. I pulled out all of the thru-bolts and hooked the bracket up to a boom. We had the boat lifted off the trailer a good three inches for about 4 hours before the 5200 seal finally gave way. The most impressive thing was it did not rip any gelcoat off the transom. After witnessing that display of strength 5200 is teh sh1t!