First of all check the harness - is it tight enough (should be tight enough you would be happy she couldn't slip out in a roll over), is it at the right height for her shoulders? For rear facing it needs to be level with or lower than. For forward facing level with or higher than. Check that she doesn't need them moving up a slot. Are you using any chest pads at the correct position as indicated in the car seat manual? And no bulky coats should be worn in the car seat.
If this is all fine:
You have two add-on options essentially in the UK. These are the two that FB group will advise.
Is she sucking her tummy in and pushing her arms through that way? If so I'd go for five point plus, you can buy them in Halfords or online.
There's also a little clip called the Besafe Belt Collector. It does the job of a chest clip but it isn't a chest clip and doesn't interfere with the workings of the straps. This is a bit cheaper so if you want to try it first it might be worth it. You mentioned narrow shoulders - if she's wriggling a shoulder free at a time, this might be better.
Don't buy the Houdini Stop, it's not approved and it can cause problems with the car seat straps spreading force adequately.
If these fail:
Consider replacing the car seat. Either with one where the shoulder harness slots are closer together, if that is the problem, or one with an impact shield rather than a harness. While impact shields are controversial, they are safer than half a harness.
You can try dressing her in a button-up shirt and buttoning it over the car seat straps once they are done up. This has to be massively fiddly - but I've seen it suggested online.
You can put her in the harness from a pair of reins, fastened with the clasp at the back. Thread the car seat straps under this harness and fasten. Fiddly and potentially unsafe if a stranger needed to get her out of the seat quickly in an emergency - but may stop the escaping.
Bribery/distraction - get some kind of screen device installed on the headrest of the front seat or back seat if she's rear facing. To act as a distraction from escaping, but also to use as a motivator - if she escapes she loses the TV.