Fusor 100EZ would work great... it's basically a 2 part plastic/urethyne mix which will not crack.. You'd want to use that to fill in all the voids - sand and then apply light filler over that ..
if there is a full thru hole then apply some duct tape to the front of your bumper and then pipe the 100OZ on the back of the bumper filling in that area.. wait 24 hours and remove the tape - sand and lightly fill to make it flush.. then off to paint.
