Two home-made couplers are shown in the attached photos; that will give some ideas on construction.
Construction details are shown on one photo. #43 ferrite will give very similar results.
Performance is also shown; they are good to about 120 MHz.
These are 30dB couplers and, unlike the ebay one, are not terminated at the Cpl port; they are used with a 50-ohm power meter or other 50-ohm instrument which provides the necessary termination.
Never operate this kind of coupler un-terminated, ie with the Cpl port open; very high voltages will be generated.
A through power of 100W (+50dBm) will produce 50 - 30 = +20dBm at the Cpl port; if this is excessive, attenuators can be used.
