Grond would be great! Thomarillion.de did a nice one in 28mm (with single wheels instead of bogies):

http://old.tabletop-hdr.de/tipps-und-tricks/bausatz-grond-von-thomarillion/

Personally, I’d prefer a scale somewhat larger than true 1/72 – like Zvezda’s siege machines. So one might use Grond also for LOTR-gaming. Since the battering ram (or wolf) only did appear very briefly in the movie (mostly the head), ‘true’ scale should be an elastic term.