https://www.youtube.com/watch?v=M68ndaZSKa8
If you look at offical Autarch models, you find capes used in combination with banners, wings or generators.
It's unlikely that you'll find an
SM cape to match the size, especially with things that stick out of the cape.
My recommendation is to make your own cape. You can experiement with green stuff - tons of "make cape using green stuff" Videos on Youtube.
Alternatively, use scissors to cut a bit from an old T-Shirt or similar thin cotton material to size, attach it at the back of the mini with two dabs of superglue, then soak the cloth with PVA glue, aka Wood glue. That takes a while to dry, but is supremely easy to handle and can be primed just like the plastic mini itself. A plus is that you can use twezers to arrange the actual cloth into folds as desired. You can bend paperclips to support the cloth in the position you want while it dies.