Make sure you have some nice BIG drainage holes there. The preferred method is to cover those holes with drainage mesh so that the soil doesn't fall through. Junipers need good drainage, so you'll want to keep the compost / organics somewhat lower (I'd go no more than 30% for any bonsai as personal preference). On the topic of drainage... while moss looks nice and is great for show, it often hurts junipers more than helps. To keep the moss alive, you'll need more moisture than the Juniper would like.
Is this a cutting or a collected sample? The reason I ask is because the height of summer isn't a great time to work on juniper roots. You usually want to reserve that for somewhere around late winter / early spring. If you cut roots now, it might not have enough energy to last the winter. It's also good to remember that when you put something into a shallow bonsai-type pot, it slows considerably in growth. If this were mine, I would at least put it into a training pot (if not the ground) to let it trunk up a bit. Everyone has a different eye for what they like, but to me, it's quite a bit tall for how thick it is. Shorter and thicker trees give a more powerful impression, so a lot of people aim for the height to be no more than 6 times girth at root base.
I would also take a look at some older junipers found in nature (yamadori):
https://www.google.com/search?q=yam...ved=0CAYQ_AUoAWoVChMIipGTya6VxwIVBSuICh0lzA-w
They tend to be more dynamic with curves, deadwood, and all sorts of movement that comes together to tell a story. You don't see them growing ram-rod straight up very often, so it looks a bit unnatural to me. Since you have wire on already, it might be worth adding some curves. This will help in compacting the tree.
Also, what is it planted in? It looks like it might be two rocks?
