use RS-485 or Normal Voltage as Input over 10 meters
If you are using master to dumb-slave single-direction long distance SPI then using RS485 buffers for data and clock is fine so, instead of transmitting chip select, why don't you gate the clock signal sent (prior to 485 conversion) instead of sending a seperate CS line? Then at the slave end you have CS active all the time.

Be aware that if you are thinking of using SPI to talk to and get responses from an intelligent slave, you might have bitten-off more than you can chew unless your clock rate is quite slow.