I equipped my first - original - trike back-piece with a handmade trunk (capacity 80 litres) made of plywood, that was built around the rear frame.
The box had a pretty good capacity; just enough to carry everything I needed on a camping holiday,
as I attached my sleeping-bag on top of it in a plastic bag.
Variation number 5: exchangeable luggage trunk on the trike back-piece.
A shortcoming was, however, that the axle and another tube crossed the trunk, thus dividing it into three smaller partitions.
That was annoying to me as I wanted to pack my luggage.
My new one was to be different: The frame should have a ring-shape, in which the box could be suspended.
(Later, this idea was also used in the postman's trike of Flevobike.):
An adventitious advantage is, that it makes changing of trunks much easier. At this moment I have only one box:
a rivoted aluminium box containing 110 litres, intended for holidays and week-end shopping.
I expect that more trunks will follow in the near future.
You can, for instance, equip a box as a tool-box that you can attach to the trike in the turn of a hand.
Or one could make a child's seat that can be attached to the frame.
Or a picknick trunk with special partitions for cutlery, plates, vacuum flask etc.
In short: ideas enough.