...mainly because I didn't know that would exist...Do you know any UK suppliers ?
Also I gave a try with the 40mm CF sleeve from EC and here is the result:

To obtain the flat bottom I've done the sleeve in 2 steps. First I have put 2 layers of the 40mm sleeve around a balsa replica of my phone wrapped in PP sheet (cut to size and closed with a hot knife). the good thing about the PP sheet is that I didn't need any release agent, the epoxy just didn't stick to it. the sleeve length was much longer that the actual phone length so that I could cut straight and flat ends. then I've laid 2 small pieces of CF on a flat surface and put my sleeve on top of that. then cleaned around the edges etc... The last part was to cut the top flat, and the put some self adhesive felt inside to protect the phone.
the surface finish is a bit raw, on purpose. I have sanded the surface down to the CF sheet an didn't put any finish coat on top of that.
I bought a clear lacquer spray from a DIY superstore in UK but it is really difficult to avoid fish eyes so I'm giving up on that.
It was impossible to hold the pattern straight (can see waves on the surface). the waviness came when I have expanded the sleeve to the right size. Any ideas to solve that ?