The howdah provided with the Eureka model is nice but too small for 15mm, so I will need to scratchbuild a matching pair for both creatures later (when I have decided which army to use them with). For a start, I decided to build the 'legs' of the howdah to match those on the BRF model (which are molded onto the model).
The 'legs' of the howdah on the oliphaunt sit on the shoulders and pelvis of the creature, as it does on modern elephants. Fortunately, two of the chains on the Mammupach also run over these positions, so I only had to making the 'legs' from balsa and putty them onto the creature's back without having to add extra chains.
Here I realized that the ears actually got in the way of the howdah. Fortunately the putty had not set, and it was still possible to bend them forward. I also had to file off part of the Mammupach's hump to make sure the howdah could sit on its legs. (If you are trying this, I suggest starting with the howdah first and then working on the other parts.)