If I understand correctly it would be an error to refer to the foot box as an "accessory". From my undestanding, never having laid in one, is that ti makes a huge difference in the positions you can easily lie in. It is a rather radical departure from normal gathered end hammocks as I understand it.
Perhaps to use the automotive analogy it is less a case of the difference between heated seats or not and more the case of front wheel drive verus rear wheel drive. They are indeed both cars, but there is a very radical design difference which is not attributable to "accessories".