This subject came up recently on the GQRP Internet Conference and a couple of great responses to external sources were suggested:
Reaching back into history is a well known article written by Roy Llewallen W7EL and in July 2021 Lex PH2LB writes:
When you want to take it a step further and measure QRPp, "Simple & Accurate Directional Wattmeter" as it was published in 2/90 QST.
He then offers his own version which has some improvements. It is a beautiful item of home construction.