Any examples of an SPI based protocol with a checksum?
I would have a look at Modbus and HDLC Asynchronous framing for examples of how to transfer packets over a serial bus. I would also have a look at this question Simple serial point-to-point communication protocol over on Stack Overflow.

Edit:

Since you are in need of low overhead I would just do what you are doing. Use Chip select to gate the packet and include length and checksum.  I would probably stay away from CRC8 as your data is byte based not bit based. I checked several SPI memory devices and none of the ones I looked at included a checksum. 

One example of a check sum is the UDP checksum. For your example that would translate to the ones complement of the sum mod 256.

For ack or status back from slave you can take advantage of the fact that SPI is full duplex and instead of just sending nothing back the slave can just send a status byte and in that byte you can use one bit to flag checksum errors. The status can also include a busy bit so you can use this to poll the slave to see when it is done and ready for another command.