I think you're using the ARB brushless single. That's a 15 amp load so you're well inside SB50 range.
I posted about making an adapter over
here.
I have since commented that a butt splice cut in half lengthwise would work as a spacer to step down the wire gauges. Failing that, heat the plug contacts and fill the pockets with solder Preheat the ends of the 6AWG wires then plunge them into the molten solder pool. Hold until solid. This will work but it's a weaker solution because the solder is providing all mechanical strength and electrical conductivity. Not ideal but you're only using it to a compressor on the ground so it's not carrying much weight. Just be sure to fill the gaps in the rubber gland on the back of the NATO plug so it's providing strain relief to prevent the wires from flexing at the contacts.
Is the ARB compressor fused inside the case? The NATO circuit is fused at 350 amps. Consider derating the circuit if you are not using it for the rear winch. I'm using a 60 amp fuse.
View attachment 7917153