RS485 transmission line high voltage protection
I use a scheme that combines gas discharge tubes, inductors, TBU protection devices, TVS diodes, and pi filters to protect my 485 buses. There are two basic schemes I use: One for the supply lines and one for the data lines. Both are shown below:

Supply line protection:



Data line protection:



These are fairly common methods to protect RS-485 buses and you will find them in datasheets and app notes for the RS-485 transceivers. 

You don't say what sort of space you do have. The above generally takes about 1 square inch per scheme on a PCB. You could put one set of protection on the top layer of a PCB and the other on the bottom side to only take up that ~1 sq. in. of space.