As you’re making a lot of parts from scratch, prototyping with cardboard or paper on each step is a great way to make sure your patterns and cuts and modifications will work in real life. Remember: gluing stuff back together ranges from ‘a big pain’ to ‘impossible’.
As with most projects, you should probably get your Raspberry Pi and custom software up and running before installing it. Testing as you go is always a great way to fix mistakes before you seal something into a case.
If you plan to wear it for a costume or at an event, do a trial run around your home during a normal day. You’ll quickly learn the best way to wear it without either damaging it or other things around your home, and find out how dexterous you still are.