There are no surviving examples of portative organs. There are however many representations of such organs in medieval paintings. One set of images that provide enough detail to make a modern copy are the well-preserved 15C paintings of Hans Memling in Ghent. Of course, these paintings tell us nothing about the inner workings of the organ but it is possible to make some educated guesses about the winding and key action mechanisms. The scaling for the two octaves of open pipes is based on the 17C 2’ stop in the organ in Staunton Harald Church (from a Harley Monograph by Goetze and Gwynn). The action is a simple pin mechanism acting directly on pallets under the keys and wind is provided by a small set of bellows at the rear operated by the left hand. The organ was designed and built over the winter of 2008 and first used during Easter week to accompany hymns sung outdoors. This is how such instruments would have been used 500 years ago.

Memling Portative Organ

Details of models available and prices, along with an order form are here.
A short sound file is here