It may not be easy to make a really good one. I had the same problem on my Bessa RF cameras and finally found a proper 6x4.5 mask for a not-very-friendly price. Once I put it in the camera I discovered that I didn't particularly enjoy shooting a 6x9 camera for such a relatively small image. The camera is just too big for that, and you can buy a teeny, tiny 6x4.5 folder like a Zeiss Ikonta or Nettar for a pittance. They make great images too.
You can probably buy a Nettar or other small 6x4.5 camera for less than the price of a film mask, and that's IF you can find one.