Most solid rails are not the same – even coming from the same stable. They can also be as someone mentioned fixed at a different angle as the roof pitch varies.
Thule spend a lot of money testing each kit to check it doesn’t come off either at speed or in a crash situation. Solid rails are an abomination anyway when fitting a rack so don’t make it even worse by trying to save 30 quid on a fitting kit that has been specially designed (it really has been) for your vehicle.
As for the ‘generic’ style racks – I think there was a thread a few months ago on here that covered those fairly effectively