Constraints for asynchronous I/O interface

What is the right way to constrain I/Os interfacing an asynchronous device e. g. asynchronous RAM? It seems to be the best way here to define min/max external propagation delay between output and input ports. set_input_delay/set_output_delay are inconvenient because they set separate delay constraints for inputs and outputs when you need only totals. And the delays are dependent on the clock period.