I think you are right Joyridin.
I think the solution is to get a resistor that is equivalent(or close) in resistance to the fan. That should give you 1/2 voltage to the fan.
To find the resistance of the fan, you would divide the voltage by the amp draw.
In your 16" summit fan example, for simplicity lets say 12.9V divided by 12.9A = Fan has 1ohm resistance.
At this point I'm not sure on figuring out how many watts the resistor would need to be. I feel like a 100w resistor would work, but the math might say 154w(or 167w at 12.9v) or 200w for simplicity. Someone that remember more electronics math than me can hopefully come along and explain. I think since you are doubling the resistance your wattage is halved. But it may be quartered across that resistor. Help....
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I'm back after some googling and at 12.9v with 2ohms of resistance, your at 1/2 the wattage so about 83.2 watts. I'm lost again at figuring out of you only need 1/2 of the 83.2 watt rating for the resistor.
A 200w is almost 4 inches long -
https://www.amazon.com/dp/B08G8JGW1M...d_i=B08G8JGW1M
Here a 100w -
https://www.amazon.com/Comidox-Alumi...77ZMFZM6/?th=1
Or a 50w and see if it smokes -
https://www.amazon.com/LM-YN-Wirewou...dp/B0728B8XQR/ 