I’m in the same boat, I can’t interlock my fingers because my chest and belly are taking up too much space. I started by just standing up straight, lifting my knee and working on locking my knee and strentghening my balance. I’ve graduated to grabbing my feet with my fingers with my knee squished into my chest however I can get it to fit. I concentrate on the idea that my legs, butt and back are gaining a lot of strength (along with my hands) and someday when I lose some inches and finally interlock my fingers I’ll be more than ready to kick out.
I’ve seen someone else in class hold their food with their knee on the outside of their arm rather than tucked in. I asked an instructor and she recommended not doing this.