If sitting on your heels is not available for you even with modifications (sitting up higher, etc), then this pose is probably not right for you. What is your intention in wanting to do it? Are you trying to stress your ankles, or the top of your foot, or ??? Perhaps there are other postures that can give you the stress where you want it, without having to sit on your heels (like bending the knee of the back leg and pulling on the foot while in Swan Pose, which I sometimes call the "Screaming Swan", or in Dragon Pose).
I guess that I was hoping to find a way to work my way into this position to help my knees become more flexible. But it may be that I just can't. Thanks.
OK so I found something that works so I can at least work on getting in that position. I have to sit on 9 inch yoga blocks. LOL I'll try taking away an inch at a time until I can comfortably sit on my heels.