BMW vs kids car seats - seat belt extenders

[Batfink]

BMW gives us lovely recessed car seatbelt buckles but due to the rear seat base moulding round the wheel arch, the kids seats drift towards the middle and push the buckles down. My kids are really struggling to get their belts done up independently. Has anyone found a good solution? Im nervous about buying a seat belt extender from china so I was going to check with BMW but am open to other ideas.