Hey Nick! Great reference! Most of what I did was actually based on this article. You are even credited in the sketches I uploaded. I appreciated but chose not to use an Interrupt Service Routine in my solution. I must have been viewing your article from a mirror site since I didn't see any of the follow up posts. Looks like I've got some more reading to do.
One thing I feel that I feel is significant in my example is controlling the MISO pin. Many of the examples I found ignored this important requirement when dealing with multiple slaves. You've inspired me to try implementing the SS as an interrupt. The biggest challenge I found was ensuring the slave had written it's data to the DR before the master begins the SPI transfer. I thought about using SS to signal each transfer and this would be much easier as an ISR.
I'm looking forward to purchasing a sensor board and SPI based sensor modules to learn more about the communication protocols used by these devices.