Hey Sac, Sounds like it wouldn't be a bad idea to rent a mixer. Or a least have a few buddies over to help mix the concrete. I mix mine with the bag hanging partially off the tailgate of my truck and my wheelbarrow underneath. I never made over 3 or 4 at a time though.
Some friends and I plan on building a lot of structures this winter too so lets take a lot of pictures of them as they are being built and putting them out. Also lets try to show the folks what the various types of cover looks like on our depthfinder screens. Especially the PVC condos.