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Simple.
Remove floor vent. Get a long piece of flexible ducting that will reach the floor in the basement and add a duct booster fan like this.
They can be had for less the $50.
Then connect ducting to your portable unit.
You could just use the duct with the booster fan to blow cooler air from the basement directly into the floor vent and into your living space. However, there would be no dehumidification. Dehumidified air is key to feeling cool. 50% humidity is close to ideal.
Make sure you also clean the fan blades on the ac unit. If you can see any dirt what so ever on the blades you could have minimum reduction of 10% air flow across the coil and air flow is critical.
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