Sash windows and secondary glazing

[Beckface]

Can anyone recommend a type of secondary glazing that is suitable for vertical sash windows?

I've looked at the type of glazing that sticks to the window frame, but if we attach that to the top sash the window will no longer be able to slide open.

Has anyone had success with another type of secondary glazing? Was thinking magnetic, but then the metal tape might be too thick and I'll have the same problem as above.

Thanks