The jacket is too thick too. The originals were quite thin, a bit of canvas over a bit of horsehair matting over some asbestos string - Vickers barrel gland packing material and tightened up so you could look along the sights.
Why do you (and many others incidentally) call the Sten a STEn? It's not! The word STEN is a word in its own right in accordance with the Oxford Dictionary.