It you don’t mind doing some manual turns, then (from memory) Road To Sky then u-turn immediately at the start. Turn right at the bottom to go up the Epic KOM Reverse, down the other side and then turn left onto the bypass road at the bottom and repeat. That’s a bit more than 3km and 5% though.
I’m specifically thinking of ZwiftMap.com and Zenturizer, which can be used to search for and examine routes. I’m rather fond of the former on account of the graphical nature - you can “scrub” through the route (desktop and tablet friendly) to see what different segments look like.
If you’re running Zwift on a PC, there’s also the RoadCaptain add-on which would let you “auto-pilot” through a route of your choice.
I wasn’t sold on NYC but several users pointed out some really good looping routes around the perimeter with the type of rolling stretches you find in Watopia’s “The Esses”. @Anna_Ronkainen had this to say:
London has some of the best medium length climbs in the game you’re looking for VO2 max efforts. Same with the gravel/paved climb coming out of the jungle, but that’s probably too short in duration.
The Volcano is too variable to enjoying doing consistent effort on it but it is around 6 minutes at full gas, under if you’re in a tough group. It starts off well but the flat middle and even slight downhill really breaks up an rhythm that you might have once had before it kicks up to double digits gradients for a flatish roll to the line. The painfully steep glass roads about “NYC” aren’t consistent enough for my preference either. I’ve done those stair steps in under 3 minutes, so that’s just an annoying world of hurt.
In reality, it would be just peachy to have some more steady 8-10 minute full gas climbs but I don’t expect it.