You can make one emperically. Since the coupling factor is probably small, size the center conductor and rectangular box for the impedance you want.
Then the coupled line is small diameter and close to the outer conductor. You can adjust the spacing to the wall and the wire diameter to get the coupling you need.
Scientifically the coupling is a function of the common mode impedance, differential mode impedance, and length. From the coupling you want you can get the ratio of these two impedances. Then from equations for coaxial and for parallel wires in a shield you can back out the wire diameters and spacings.
There is the question of do you really need a directional coupler. It has the advantage of making the driving pulse lower amplitude on your receiving circuit but the disadvantage is the rising frequency response which will distort the pulse shape. You could easily put your receiving circuit directly across the line and have some form of protection circuit, like a diode shunt, to keep it from being damaged or saturated by the driving pulse.