Here's the last pic I took of him in his bed and one of my favourites that I took of him with a close up lense
I'm sorry for your loss. You did something very right to keep him going so long.
To bury him, just dig a hole in your back yard or garden. You don't have to go down far if you cover the top with heavy rocks or wire so an animal can't dig him up. You don't need a box or anything, but for me, I have MAJOR grief when I throw dirt directly on an animal I loved.
I am sure from your description of Leon's last few hours that he probably was too out of it to notice, but the next time you have a chameleon that is dying, please just put it somewhere safe such as nestled around the base of one of the plants. Put him somewhere where he won't fall and will feel safe hidden in cover. Dark is good. Or put him in a small dark box with wadded up damp paper towels in it. Chameleons are not pets that need or want any emotional reassurance or support. From Leon's point of view, wrapping him in a blanket and taking him out of his cage would have been akin to him being wrapped in the jaws of a predator, which is not what I think you wanted him to feel as he lay dying. Wrapping him in a towel was comforting to you, not him. As I said before, I doubt he was all that aware anyway, but I can't let that picture go without commenting.
I'm truly sorry if my comments have caused you pain. That is the last thing I want to do to someone who truly loved their chameleon and obviously took very very good care of him since he lived such a long life. This is written for other chameleons and their owners.
Again, sorry for your loss.