Here is a top-view diagram of the frame and it's components.  The uprights are not shown for clarity.
 
 
 
 
 

Here is the frame with the front stretcher, rear stretcher, and the middle crush bar installed.
 
 
 
 
 
 
 
 

Just an idea of scale - this is a heavy and large engine, and will require a few people to move it.  Onagers of effective power and size were not highly mobile, but could be moved with the effort of a reasonable crew.