What are some design concerns for off board SPI communication?
Carefully consider your grounding scheme.  Shield the data lines, if it helps, and ground the shield properly.  Don't run data and clock on the same twisted pair.  Use galvanic isolation if needed.  Other than that, there are no rules of thumb that I aware of.

SPI was designed for (1) short range communication, usually within one PCB and (2) in the environment with not a lot of EMI.  Perhaps, the only buses that perform worse than SPI in presence of EMI are I2C and 1-wire.  There are buses, which were designed for long range communication in the presence of EMI (RS-485, CAN, Ethernet).

It's possible to extend and ruggedize the SPI.  Here's an application note, which shows an SPI bus with differential lines.