I'm not sure if there are two versions of the middle/box splits. One where you have toes pointing the ceiling and one with toes pointing forward with the feet on its side.
I've been practising the toes-pointing-at-ceiling version for a long time. I use a guard rail or a metal fence to practice the splits on where I can thread my arms through the metal bars and grab it and pull my pelvis closer and closer towards the guard rail. Love it.
For the toes-pointing-forward version which I'm struggling with it. I feel like the muscles on the outside of both legs are getting crushed under my hip bone than feeling a stretch. I don't feel much stretch elsewhere on my legs, maybe the pelvis/groin area... Could I be doing it wrong or something?