I have not had to actually make a back yet, so far I've been able to adapt backs from other cameras. You didn't mention what format this camera was originally designed for. If it doesn't have the back I guess you may not know, in that case I guess you can choose the nearest size. What you need to know, in any case, are the outside dimensions of the body. Then you can look for one with the same, or similar, size back.

I have an Eastman 2D 5x7 and have found that the Burke & James backs are the same size, but the pins which attach it to the camera are located slightly off from the Eastman. It's a simple matter to fill the old holes with a small dowel and relocate the pins. I use brass rod from a hobby store to replace them.

So, there's your next question: by what method did the original back attach? It probably uses the pins like the Eastman and the B&J, but there's always the chance that the original back was one of those old hinged jobs. Really old. Haven't had to deal with those myself. In that case you may want to scrounge up some new hardware to allow the more common pin-type attachment.