I would first look on ebay to try to buy something.
If I could not and had to make something, I would take two of those 500 watt beo resistors and connect them in parallel as a 50 ohm load. I would mount them on a rectangular copper pipe, and run water cooling at a high rate (like a garden hose running full on) to keep them cool. I would then capacitively sample, with a small 1000v ceramic chip cap, some of the power being dumped in the 50 ohm high power load. Select the capacitor for around -20 dB of coupling. After that, I would place a coaxial 1 watt pads, say 1 dB, 3 dB, and two 10 dB pads. Then I would carefully turn on the transmitter, slowly ramp up the transmit power, and see what came out the end coax pad.