It turns out that the wall thickness on the cooler I'm installing bulkheads into are not as thick as the manufacturer states. What a surprise, huh?
Good thing is, there are bulkheads available that can manage the reduced thickness. In this case, Hayward has some PVC bulk heads (BFA series) that can accommodate thick walls:
These should work for my application as is.
Thanks everyone for the help and the great ideas!