Its in all the old books, connect a 100 watt incandescent bulb to the transmitter....well here is one.  I tested it and it works well.  Please note it barely lights at 100 watt (you could run much more power into it I believe).  Someone made this decades ago, probably the 50s.  Pretty cool piece of ham radio history.  Has a UHF connection (I include a female adapter).  The bulb dummy load is especially effective with vacuum tube transmitters.  It also will indicate modulation for an AM transmitter.