How should I fix this hole at the top of the sink

That's a drum trap, not a P-Trap. Be sure it's listed to comply with your code requirements. Looks slick anyway. You'll need a 45 degree chrome compression ell and maybe some extra pipe. You'll end up with the trap arm out of the drum running at a 45 degree horizontal angle to the wall. It will be cut quite short before it enters the ell. Upon exiting the ell, it should be a straight shot to the wall connection. The remaining trap arm you cut off may or may not be long enough for this. It then ties into the wall pipe with the same adapter fitting you would normally have used.

The trap arm runs entirely horizontal, despite the ell, with just enough slope to drain (1/4" per foot). The only significant vertical drop is in the tail piece that drops straight down from the sink. If you prefer the 45 degree segment to be closer to the wall, you could do a variation using two 45 degree ells, such that the trap arm out of the drum goes straight back for a distance before making a dog leg to the wall drain.

I'd use the square-walled for the sink, so you have more room to work with for pipes. And using the one with cylindrical walls as the main cabinet will help keep items from falling off the shelf into the nether regions the square corners would leave open.