The FMS FCX24 cassis has a two-speed transmission.
The transmission rod leaves on one side of the transmission for the servo and on the other side to manually lock it to one gear permanently.
The side to look it always works some sand into the transmission.
With this *.stl the rod can be covered and the sand ingress minimized.
I think the FCX18 uses the same transmission, so this should also fit there. Maybe someone can give feedback so I can update it later on.
The author marked this model as their own original creation.