Ratio of canvas-top to open-top trucks.

[Legbreaker]

I'm with Rae on this. Depending on what the vehicle has been through, chances are there's not going to be a lot of standard issue canvas left around. What is left is likely to have been patched up and/or modified to some extent.
Where the canvas has been destroyed/lost/whatever, it's possible alternative covers may have been constructed out of local materials. This could be anything from plastic sheeting, animal skins, or even perhaps salvaged roofing iron.
T2K in my opinion is a real playground for diorama construction - almost anything is possible.