You can attach the Bob to a "nutted axel" using their "Bob Nutz."
I'd like to add that the pin used to attach the Bob to the bike is extremely unlikely to fall out; see their web site for its geometry. I've used it extensively over the past few years, unattaching many, many times and never had one of the two pins come out unintentionally. On trips I always take a couple of spare pins in case I lose one; hasn't happened. In an emergency, some thin wire or a safety pin could probably substitute until something better could be rigged, if one were to be lost and no spare.
Others commented on "bouncing." I've had to ride on some pretty rough roads, including a long stretch with rumble strips. Not fun, but the trailer behaved better than my bike.
As to other designs, I can only speak to one rider I rode with who used a two-wheeler with, IMO, poor structural support for the load. He had some major mechanical problems with the design, as well. In addition, he had to ride farther out from the side of the road due to the dual wheel design. He had a number of instances where the inner wheel would drop off the hardtop, which caused some concerns.
Hope this helps with your decision.