My favorite high current 12V load is auto headlamps average sealed
beam is not less than 80W often as much as 120W so two are easily a
10-20A load. Some of the halogen lamps are higher power but get very
hot (caution).

the key thing is enough load to simulate a radio and parallel lamps
of sufficient power will do that safely.
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In the absence of car bulbs , a good dummy load to test a power supply
unit is to take a length of galvanised fence wire of say 1mm dia or larger.

For a 13.8V-20A supply the load resistance needs to be 0.7 Ohms.

Send a current of 1 ampere through the wire and measure the voltage with
a DVM. Subsequently slide a crocodile clip along the wire until the
voltage measured is 700mV ,then you have the correct length.
Coil the wire and connect to a "chocbloc" connector.
Place coil in a plastic bucket filled with water.
Although tap water is somewhat conductive the overall resistance will
probably not decrease significantly. If it would you could make the wire
resistance somewhat higher (longer wire).

An alternative method would be using a roll of PVC insulated copper wire
of sufficient diameter determining a 0.7 Ohms resistance with the above
method. Again use as dummy load placed in a bucket filled with water .

Frank GM0CSZ / KN6WH