Running into a game design problem D:
The cube moves way too fast for the player to have any chance to put portals in its trajectory. I tried a quick hack where I let the player slow down time, but I find myself just holding the "slow time" button, which takes out a lot of the fun of seeing this cube zooming around. You can play around with it here: https://maxbize.github.io/portal_deal.html
In traditional Portal, this isn't a problem because you just need to look forward to anticipate where you're going to hit a wall - here we're playing from an "observer's" perspective.
To solve this I'm considering allowing the player to set up a chain of portals at the start, then hit go, and watch the madness unfold. They can go back and tweak their chain until they find a combination that lets them beat the level. Anyone have other ideas?