A long time ago  Bear archery made one just like that.  It has a filled plastic or glass platform and the bracing was below.  A chain held it in place.  Once you put it in place, it never moved.

Problem was, the spikes penetrated the tree,  big time making it illegal to use in public grounds.
ChuckC

Yes the brace adjusts for leaning trees..

You wouldn't happen to have a picture of the Bear River stand you are talking about ???

I'm sure it would be easy enough to make..