https://lowkey.gg/v/86c76a56-b344-45cc-ab41-6d5558c4a2a6

I found a way to solo boost with any agent on top of the box in C site. The jump utilises the crouch bug and the jump is much easier to perform if you bind crouch and jump both the space as it allows you to spam jump and crouch much faster.

This is what your binds should look like

I find that with this jump the further along the ledge you get the easier it is and you should also use both W and A when strafing into the box and hold those keys while spamming space bar. An added bonus of binding jump and crouch to the same key is that your jumps will always be silent and you can also full speed bhop up slopes, which is far better than the old method where you walk and bhop for faster yet silent movement up slopes.

Edit: In my opinion, this is the hardest jump in the game so it will take practice to get good at it.