Sam,
Have you received an answer to your Question?
Here's My 2 cents.
I believe that petsmart has crates that they donate for PNP.
Get crates that are comfortable for your plane and remember that one size does not fit all. You may have to disassemble the crates to get them through the door and reassemble in the plane, and I have a few cheap lightweight ratchet straps to secure.
You will need to check your W&B with the rear seat removed, seat weight should be in your POH along with the arm.
In my Saratoga with full fuel and a front pax I have to watch my forward CG.
It has been my experience that while the ground crew will tell you that they transport fine in the car doesn't mean that all will be fine in the air.
I've had numerous times when the rescuers didn't like the idea of the crate

. If they only knew what kind of mess the animal 's left in the crate that I had to clean up

!!
So, be prepared with a little cleanup after drop off. I usually use a several layers of old newspaper that's wadded up afterward and put in a small trash bag for disposal and a spray cleaner with paper towels (wouldn't want to deep that smell with you on the return trip

), and a pair of latex gloves that I keep in a ziplock for the removal of the pets (If the crate is messy so are the pets, which you'll usually find out after it's all over everything

) and the cleanup. But I've found It's mostly the pups that leave the mess and the older pets will usually be fast asleep after 15- 20 Min. Or the controllers find it funny when they hear barking in the background

.
Some of them don't like more than a certain amount of animals in the crates. I just explain that those are the seats that are available

.
Happy Flying
Leonard Guidry