I would assume a lot is down to the quality of the pantograph.
I understand that a panto was a standard piece of industry kit, so they must have worked.
I have tried to use a kids (toy) panto befor and it was crap. the reaon being there was flexing in the arms.
These arms were made out of the same kind of plastic as balck plastic coat hangars you get when you buy a suit.
The inertia of the pencil dragging meant there was a lag in pencil action relative to the movement of the 'pointer' on the original drawing. Then the pencil would race to catch up causing all sorts of issues. This was made worse by the joints being stiff, more inertia. the pencil squeezed into a rubber grommet, another source of flexing.
If it is a one off, you can make a panto to meet your scale needs. I imagine that wooden slats would work well, but they must be thick enough not to twist. And there should be minimal slack in the pivots at the joints, but they also must not bind.
This is a link to a standard panto, but it will only really do drawings of up to 350mm. I think you may want bigger.
http://www.clp.co.uk/product.asp?Prd=020593&$$tid=JHPgipMb2yW_NnVBkG1vxExwzDWIhfv3j198upgeZvyjfJuHAwGAttkDtWlfYfKR&=