"Bar Bait" (brand name) intended for Norwegian Rats.
I use it and it works. I keep a block under the hood and wired down so they cannot remove it. This site lists several good baits.
Poison site Make sure the pack rat and/or bait are not going to wind up where a domestic dog or cat will get to it.
It makes the critter thirsty so if you have a bowl of water nearby they will go there to die.
I put a water dish under an upside down planter with a piece of PVC pipe that is big enough for the rat but to small for a cat.
I cut a hole for the pipe and put a weight on the top of the planter.
Check the planter every day for dead or dying critters. A big pack rat looked dead to me one day, but was still alive. I went to get a shovel and came back to find he had started to crawl away, so keep a shovel handy to bury the dead and dispatch the dying.