In article , Janet Tweedy writes
Having looked at your pics and having done this many times before, the solution is (as a few have said) to drill the top part of the plastic off, leaving the remainder below the surface and then fill over.
As you can see from this example, the drills are available cheaply:
Btw, that company has free delivery on orders over £10 so buy some filler and a filler knife and you will be there.
If you don't have a suitable drill then hire one, it won't be a difficult job but the bit may grab a bit as it cuts so not one for those with fragile wrists.
If a builder (or handyman) couldn't do this in 2 mins then find another ;-). They may not have a bit that large though so it wouldn't do any harm to get one in and save their markup on it.