I ordered some 200mm castor wheels from Western Australia and have attached them to the Outback Shack. I bolted them to lengths of reasonably solid timber which in turn were screwed into the base of the shelter.
To attach this setup I needed to lift the shelter onto one of its ends. I managed to avoid death whilst doing this and remain hernia free. The screwing of the base plates presented some hurdles, but after rummaging around the garage I chanced upon some better screws and all is well.
I splashed some paint on the under carriage of the shelter and it is now resting on its new wheels. A happy side effect of this process is that it has lifted the micro shelter a further 30cm or so off the ground. This makes getting in and out much easier and improves the view from within.