Hi,
I had a friend who had a Fraser 98 mauser years ago in 303. I don't know whether it was a velox or not, but he used to turn the rim off 303Br cases and cut an extractor groove to make what was essentially a rimless 303Br.
I think I still have a round somewhere, so maybe able to give some dimensions if needed. This may be a good place to start and would be an easy source of cases.
LesLeeSpeed