Topper Repair

I have a topper sailboat whose transom plate has been damaged and needed to be removed. The problem is, there are now holes where the screws went in that need to be filled in before I can screw in the new plate. How do I fill in holes in the plastic?

I think the transom plates changed but the mounting holes were the same and they have a captive nut set inside the hull.

Originally posted by Buzz Yes to replace the transom plate you usually cut a hole in the boat so that you can get at the captive nuts with your hand. Then you weld the boat back together again with a plastic welding gun and plastic welding rods.

No, don't do that.
When a captive nut falls off inside use wire/fishing line through the bung hole to pull a new M6 bolt from inside.
The OP says he changed the plate and has unused holes, this is only possible if the hull is a 'pre-production' moulding with recesses in the transom to take the over complicated transom assembly.
Unless we are talking about the 2 rivet holes that hold the inner plate?
Perhaps the OP could clarify?

Assuming this is a later boat with a flat transom, think the moulds changed in the late 1#,###'s
The nut plate is inside so you wouldn't see it.
If the screws you removed are other than M5 machine threads then the boat has been previously 'repaired' and you are in uncharted water so to speak.
There should be 5 x M5 screws + the 2 screws & nuts at the top.
It is quite possible that the screws have been fitted with sealant by previous owner(s)
But I can't explain why the holes are oversize, they shouldn't be.

If the holes need filling you probably need to get it done properly as it is unlikely anything much will stick to the material as it is a type of polythene IIRC. Boat yards that fix toppers (and trilam/rotomould boats) have a special tool similar to that used for vinyl flooring to heat the repair strips and melt them in.