Sometimes an “4m antenna equivalent” can be bought on various auction sites (the labels are in one of the Warsaw-Pact members’ languages, often russian):
This is an rf thermocouple ammeter (3A or 10A range) in series with 10ohms and 68pF (both sized to handle an adequate amount of power). This was used for silent tuning of TX output stages.
This is a very good source of cheap rf ammeters. I suggest connecting the input (labeled вход) directly to the rf ammeter terminal.
For searching auction sites either use “4m antenna equivalent”, “ekwiwalent anteny 4m” (polish) or “еквивалент антенны штырь 4м” (russian).