If the front leg is straight, it can just bend upon contact with the floor.
I, as well have been teaching the basic lunge and hurdle with a very deep lunge, if flexibility and strength allow. The back and arms are at the same angle as the back heel. Roughly 45 degrees. I exaggerate and show 30 degrees but this is just to give an idea.
Their knee is over their front foot and the chest is over the knee. I will exaggerate and place my chest on my quad of the front leg. However, this is requires a lot of flexibility and strength to move through. It just gives them an idea because if they cannot move out of the lunge, it's a moot point even if I teach them a deep one.
I often use an example that the lunge is from fencing and is a vicious thrust forward. We will often do push to ball of foot, fall forward and hit the lunge position and not lose the proper tight shape. Too often too small of a lunge can really muck up the CW, RO, FHS, and basic kick to HS.
I do notice a lot of elites do hurdle high. I do know most are so powerful, there isn't the need to try to maximize how much power to create from a long leaning lunge/hurdle. It often looks as if their hurdle just floats in the air then falls and their tumbling is a blur.
Still for beginners, I will probably still work what I'm doing. Maybe at higher levels and strength/power levels I'll consider changing it over to the high hurdle. I need to play around with this myself.