Glue a section of plasticard around the inside edge of each coach half, then smooth the front and rear edge of the extension.
BR MK1 MK2 FK3d COACH ROOF VENTS (x24) - TYPE 2 (OO Gauge 4mm scale)
Hornby OO Gauge BR MK2 TSO Coach Standard 2nd Class Open Coach Seating/Interior
Hornby OO Gauge BR MK2 BSO Coach Standard Brake Open Coach Seating/Interior