The lower half of the refelector should be painted with matt black paint as well, but that's perhaps taking things a little too far !
The 1938 'Mechanized Movement by Road' training pamphlet actually referred to an opaque cardboard mask and it seems from photographs as if some early war vehicles actually had masks made of the brown envelope paper that we would normally regard as 'manila' At some point this became the thick black card like Henk's NOS mask.
Cheepest option is to paint the inside of the glass blue as per BEF instructions but there is the risk of a pull for impersonating the rozzers !
I just send you the info on the black out masks. The one with the measurements is the original one I have myself. For the early BSA's there was also a small black out mask for the MT110 tail-light, I have send you these photos also.
You can of course also make one from an old Shell oil can.