Deadeye: Oh, why didn't I realise that when I was making the original version X). I solved it later, but used totally different method. Oh, and btw, thanks for the great tutorial series! I'm sure that this will help many people awfully lot on their way of learning Construct :D!
I made a version with slopes of any steepnes. This version uses some trigonometry (to calculate the position of Charlie related to the hitbox in slopes) so it goes ot of the scope of this tutorial, though. The positioning of Charlie doesn't actually look very good, but currently that's impossible to fix. It would recuire some fundamental changes to the engine. (The hitbox's corner should be able to be "inside" the slope)
However, you can easily modify the original to add slopes with different angles.
Here's the new version: