Mortis Engine seems like it'd be the best fit...just say the relic is being carried in the howdah, and there you go. And it being a chariot (with impact hits) feels right for a mammoth, to me.
Out of curiousity, where did you get the mammoth model? I wouldn't mind running out a mortis engine someday, but no way in hell am I ever buying the GW one; it's too expensive for a model that I hate.