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.
_________________________
- the sun and the sand and a drink in my hand,with no bottom...no shoes,no shirts,no problems.

- no boss, no clock, no stress, no dress code...no shoes,no shirts, no problems. - Kenny Chesney -