I saw a video of someone doing the Haven C site box jump earlier today and decided to share my method of getting on top of it. As far as I know this can be used on a bunch of other spots on many different maps, I've even replicated this on Haven's A site as well. This method is also a lot easier with Jett, and is useful because you don't have to waste an updraft. Just have your teammate crouch in the shown spot (I don't believe it matters what agent), and jump on top of them while spamming crouch. It can take some finesse, and it might take a little practice, but it's very easy once you get it down.

https://reddit.com/link/wr14ad/video/oiapyd0pfci91/player