The instructions in the score for the Mahler Box is simply him requesting a short, strong, dull, hammer of NON-metalic sound. I've heard it is supposed to resemble th sound of a head being cut off with an Axe.
You could simply get a box made from good quality wood. Make it about 14 inches on top and bottom and about 24 inches high. Cut a good hole, or several holes for resonation. Then you can also make a gavel out of wood of any size, or perhaps use a rawhide hammer. That should do the trick I think.
(Or you could loosen the tension on one of the tympanies excessively until there is no definite pitch and hit that with a firm tympani mallet.).