I'm trying to figure out how to use NonGNU ELPA packages from straight.el, and it doesn't seem particularly obvious. Actually, maybe it's not even easy or possible without further development, since:

• for GNU ELPA support, the https://github.com/emacs-straight mirror is required
• there are dedicated backends for GNU ELPA / Org ELPA, with code such as straight-recipes-gnu-elpa-retrieve
• there is no such backend for other ELPA repositories

Currently there are only 6 packages in NonGNU ELPA, but at least two of them are probably fairly popular packages:

• org-contrib, which has been recently removed from Org master branch and will not be distributed within the upcoming Org 9.5 release
• markdown-mode

and I expect this list to grow over time. So it would be great if it was easy to access these via straight.el.

Even if the maintainers don't have time to add support for this soon, if any hints could be shared here on the rough approach then maybe I or someone else will find time to follow those hints at a later date. This reminds me of #686 ("Confused about mirroring of GNU ELPA packages"), the resolution of which might help with understanding how to solve this issue.