That is a great Mumak project! I had selected those same elephants for my Mumaks about 3 years ago. The first Howdah I built out of wood and did not like it at all. I did buy 3 of the Mithril Mumaks, just to use the crew figures. I also bought additional corsair archers and spearmen to increase the size of the crew.
I would like to find something like you have for the howdah; building my own is what has delayed the project so much.