A few years back I did a dummy load in much the same fashion Jack did, but using ten pieces, 510 ohm, 2 W resistors. Used flat copper flashing strips instead of wired to mount the resistors. Intended for QRP.
I used a 1/2 pint paint can from Ace Hardware. Poured the paint into an olive jar for later use and cleaned out the can.
Named it "Cantennino" - little cantenna ?? - as homage to Heathkit
For 10W and under, there was no need to use the oil fill.
Jack's detector and Arduino system should work just as nicely on the smaller load.