You could also use worbla. Worbla is pretty stable for things like these. With worbla, you can easily create 3-D designs like this. http://www.worbla.com
(Example: This headpiece was made only using worbla: http://blog.cosplaysupplies.com/wp-c...dressfinal.jpg
) Someone already made a belt that is almost the same pattern as the one you showed by just using worbla: http://www.worbla.com/?p=358
If you never used worbla before, I recommend buying the sample size (the sample is only $18 in US, and $22 in Canada, price varies everywhere else though) so you can get a feel for it. Then buy the size that you think you could need. I've never worked with Wonderflex, worbla, or anything like that so I was skeptical about buying it, but you can learn how to use worbla really fast and it's extremely easy to use and mold to shapes. It's much easier than glue and clay, trust me I've tried them all.
As for attachment, well that depends on what you use. If you use glue and clay, you would definitely need supports. Then attach the supports to the shirt and shoe itself. But if you use worbla, it stands on its own. There's no need for supports, but redundancy can never hurt a cosplay, so you could use supports with worbla. With worbla, you can mold it straight to the shoe (depending on the shoe fabric) or make a buckle and attach it to the buckle. Same goes for the shoulder one, but I'd mold it to your shoulder first, then attach it to the shirt or make a buckle for it.
Good luck on your cosplay!