I dug through the pile of "spare parts" and found another Canon T-ring. This one is 11mm thick but the thread size is 48mm and the flattener takes 42mm. Clearly not all rings are made equal. Using 11mm would give me exactly the needed 55mm distance.
I also found two Nikon rings (48mm and 42mm threads) and they were both about 8.5mm thick. For Nikon we have 46.5mm + 8.5mm = 55mm. Spot on as required by the flattener.
I will try my Nikon D90 once the weather clears up. It has been cloudy for days and the forecast does not look good at all