Yes, those are some homemade stirrups that are looped onto the strap that attaches the web aider to the tree.Nice. What's the arrangement that you have your feet in? It looks like some straps that are strapped to the tree?
I like it. Now just double up the ply on the whale tail so you can drill out a hole for a cup holder. Right through the middl of it.
are they comfortable? Can you stand in them for a few minutes to rest from sitting?
as a pee cup or a drinking cup? either way works just don't forget.