If I don't have a back strap what's another effective way to scale down SE16, SE17, and SE18. I'm slowly realizing (though it's nothing new to me) how to better engage thoracic extension and work mobility/flexibility in a healthy, restorative way and that this must be imperative for the health of my shoulder and my handstand work. So, I need to dial these down, way down (with the wall walks it's apparent that I just don't/won't go as low and will have to focus on stretching the upper back if that's even yet possible), but seeing as I don't have any ledges high enough in my apartment upon which I can raise my legs, I'm thinking of doing them as the scaled down images present from a standing position. My issue, though, is that I'm not sure of how to do this due to my lacking a back strap. Your insights are appreciated. Thanks.
Edit: I mean to say SE 16, SE 17, SE 19, SE 21. For SE 19/21, I believe that the scaled options should worst best (and will approach SE 23 from a similar position).