I've made a couple of these, and they're really fun. They're a bit fiddly, but if you've successfully made wreaths or other topiary, they are pretty easy.
The first one I made was a round wire topiary ball, which was certainly a steep learning curve to deal with - I eventually put burlap inside and filled it with Sunshine mix #4 (with the extra aggregate). I poked holes or cut them with a razor and stuffed the bareroot Sempervivum in to the holes. Some of them kept falling out so I wrapped the roots in newspaper and then they pretty much stay put. Now they are filling in with chicks, and seem to be rooted. I'm not sure how it will over winter, but I'll post more information on that topic later.
The second one I did was similar, but was two coco fibre wire baskets stuck together after filling them with the same mix. This will be better in the long term as it has more soil, as well as bigger. However, it is also lots heavier, and if it ever falls it will kill me!
The picture shows the big one with the first one (smaller) in behind it.