How wide is the gap? 1cm more, less?

Silicone sealant won't be any good for that sort of gap, I don't think you have any other option apart from strong tape such as waterproof cloth gaffer tape or similar. It might be worth trying to get a bodyshop to try and get the gap a bit tighter, even if it's a quick and rough job.

You might possibly be able to do something with expanding foam. If you mask the outside of the wing and lamp assembly with tape and spray from the inside it should seal the gap that you have.

A few photos of the damage might help us to give you some more ideas. From your description, OP, it's very hard to visualize, I'm afraid.

Why not try body filler to bridge the gap? Easy to use and maintain the shape

Probably what I would do in the UK, but AFAIK you cant get it here (in Taiwan).

Rust holes on the Skywing were filled with some kind of mastick bedding compound when I bought it. This is less likely to encourage further rust (filler soaks up water) but I'd think more difficult to apply to gaps. I didn't put it there so I'm speculating.

My local improvisations for this sort of thing are

(a) Chewing gum. Works OK but you'd need an awful lot for the gap you describe and your jaws would really ache by the time you'd finished

(b) Crumpled and compressed aluminium foil wetted with sunflower oil. This can be pressed to rough shape and then sanded. I've only used it on a couple of small holes, though, and never used it on a gap as big as the one you describe.

If you use filler, try and waterproof it well before painting. I'd use a vegetable oil (linseed probably best) or an alkyd resin like Penetrol

It would be easier to use Gorilla tape, just cut to shape, place in and stick around gap wait to harden, its waterproof as well

I wouldn't normally recommend this type of repair but have seen better body finishes with tape than using fillers on small areas if done right and hardens within the hour

Not seen that. Perhaps it didn't exist when I was banger-bodging in the UK.

People (myself included) used to use actual fibreglass to fill / bridge big gaps and holes, which sounds similar but less convenient.

I can remember putting some really quite big patches on the rot traps in the inner wing of my Marina, filling the cavity with bonding plaster (or was it called "browning"? Can't remember now) which had expanded vermiculite in it and could be easily shaped, then removed once the fibreglass patch had set. Wasnt intended for the purpose but worked quite well.

I'd guess no one does that kind of thing to cars nowadays. I originally bought the fibreglass to fix a dinghy.

Half a centimeter of clear silicone sealant versus maybe a couple of centimetres of fibreglass, seems a pretty fine (aesthetic?) distinction to me. Filler does tend to promote rust but it can be painted, which silicone sealant, AFAIK, cannot. Appearance doesn't bother me but it does most people

Tis, however, true that nearly all my cars have been obviously near end of life when I got them and for all the time I've had them, 8 years in the case of the current one.

The exception (apart from company cars which don't really count) was a fairly new and shiny Renault 5 I took over when my mother stopped driving. Burst into flames a couple of years later.