I read all the info I could find for it before buying it and the first thing I did was empty and flush the cooling system, I removed the thermostat and run it for a couple of weeks like this and then flushed it again and replaced the thermostat, loads of crud was washed out and it has run as 'normal' since then, I know the thermostat is working correctly as the return hose warms up as the water flows around the system. My only gripe is that it seems to take a long time for the thermost to open and the coolant to start flowing. A lot of folks on the forum say that the heater matrixes do the majority of the cooling and the thermostat rarely opens but I do not subscribe to this.
I would imagine that the Mazda Engineers got it right or they would boil all the time, but a bit of tweaking here and there would not hurt so after months of looking for info, talking to others who look after their own vehicles, not just Bongos, I was taken by the article I mentioned - see
http://www.mgfmavhh.ukf.net/ -, read it and see if you can make sense of it.
I have a mate with a workshop and I may at some point ask his opinion about fabricating a housing and see if it is a feasible project, unless we have a budding engineer willing to take the job on and let us have any feed-back - it may be the way forward, who knows?