Summerscales Army Surplus
http://sas-surplus.co.uk/ has the canvas covers. Found them via eBay last year and I bought three covers in a fixed price listing. They were three for 5 GBPs at the time but now it looks like they are getting that much for each one. An example of their current eBay listings can be found at item number 360123135023.
I think these covers were for bore brushes. They are a Number 4 size which is roughly 4" diameter. I've seen Number 2 size covers coming out of Australia lately which people are selling under the guise of being 2-inch mortar muzzle covers.
For the 4" smoke discharger, it takes two of the covers to build one which will fit since the inside diameter of the cup is 4". Essentially you need a bigger diameter end piece and about another 1.5" of side material to create a proper looking and fitting muzzle cover. The examples I received all had very closely matching canvas and all were 1943 dated. I made the new end disk from the donor cover where the markings were located so the final product seen on my launcher has nice markings on the side and on the end.
Not sure what was really used during the war but would guess it was a shorter leather muzzle cover similar to the type used on 3-inch and 4.2-inch mortars. I've got examples of both of those for my mortars and they won't fit the smoke launcher barrel as one is too small and the other is too large.