Pet Carriers for Rad Rover

[RCGoodin]

I have a new Rad Rover coming. I also have two medium size pets that I would like to pull behind the Rad Rover. The problem is most pet carriers attach to the side of the rear axle. Rad Support said I cannot use a carrier that attaches to the rear axle, it needs to attach to the seat post. I cannot find a carrier that does that. Can anyone help?