Following on from the initial post, I have just replaced the first lens, and it fits perfectly, doesn't leak, and I'm really pleased with the way it looks.
As said before, these are to replace the old lenses but these are fixed lenses (they do not open). If anyone is interested, I have 14 of these extra (I had to buy a whole sheet of material), and I can provide them with screws, caps, and some fitting instructions at £40 each, which is about 1/4 of the Lewmar price.