just slightly wider than a 1x2 foundation. inside the box there's an extra wall on the left side, that was the only way i could get it to work consistently

NOTE: from my understanding these hyperloop circles are very FPS dependent on how well it works. so depending on your FPS it might not be as consistent

IF you get stuck inside, don't worry, just run forward into the entrance and it'll shoot you out, you might have to play around with it a bit inside, like try quickly run sliding towards the left against the wall and it'll usually work