Sheetrock over 24-inch centers
 

Another option is to double-rock it with two layers of 1/2-inch sheetrock.

Any suggestions as to what would be the best way to go with this? --
5/8-inch, double 1/2-inch, something else?

SOMETHING ELSE: They make a STIFFER ceiling board..get it from a REAL
sheetrock distributor..not BigBox store