I bought some light-fittings from Migros (pictured)

, which generally work by having to drill a small bracket into the ceiling which the light fitting then screws on to.
The problem I have is that the ceiling cavity is 50mm wide, yet the screw holes on the bracket provided with my light fitting are only about 25mm wide. Therefore I cannot fix something to thin air! The hole that the wires are hanging through is a kind of grey plastic piping (50mm wide), so I preume it's some kind of swiss industry stadard as it is also a brand new apartment from a big company
Does anyone know what solutions there is? The lights I bought are from Migros (the turbo spotlight range) so I presume I'm not the only one with this problem!
I think I am looking for something like a 'celing dry lining box' (pictured below) or maybe a kind of plate that fits over the hole that I can feed the wires through and drill the bracket into. However the Migros electric place doesnt have anything like it.
I suspect there may be some widget, perhaps used by swiss electricians etc that can help in this situation. Can anyone help?! Thanks!