I am going to try to make a semi for Dec.2.

It is a flatbed trailer. How would any of you recommend to make it ?

Should I use the loaf pan, and trim some off to make it not as high, or is there another way ?

Have you seen Wilton's Fire Truck cake pan? I was asked to do a fire truck birthday cake for a little boy and bought the pan. Inside on the instruction sheet it had different suggestions for decorating: fire truck (of course...), school bus and a dump truck.
Just an idea....

Forgot to mention, it is going to be a 3d, so it will be sitting up.

Thanks for the idea though.

This is instructions for a fire engine, but should be easy to adapt.-scroll down...
http://www.reasontoparty.com/firefighter_party.htm
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Here are instructions for a truck that may help.

http://www.wilton.com/recipes/recipesandprojects/add_pans/deliveringgoodwishes.cfm