Dan,

My previous edition used the decks, rather than the spinnaker pole, for vertical support. Here is a not so good picture:
cargo tramp
and here is another:
Whisk on the rock beach

The trampoline was a trapezoid shape; a rectangle would be fine also. The back of the cargo tramp was supported by the front cross bar and the front of the cargo tramp was supported by a carbon fiber fishing pole base section. The carbon pole spanned the decks and was wraped with rubber hose to protect the decks from scratching.

The forward pole was held in tension with low stretch line that ran forward and attached to the fore stay bridle wire tangs. I did use some support on the center of the pole, up to the spinnaker pole. But I think that you could run a line from the forward pole center, up to the top of the fore stay bridle. This line would not be needed if the forward tramp pole was rigid enough to suspend the cargo load without much flexing.

GARY