Before making a full plate back you should be advised that 6.5 x 8.5 holders come in a few different sizes. I have 4 of them and 3 are for century and the other I had to modify as the outside dimensions were quite different. You should also note that full plate film holders are almost unheard of. If you want to shoot full plate you'll most likely you'll be using glass plate holders and either adding a sheet of glass or modifying them to take film. I got super lucky and my full plate camera came with one film holder and later on I found 4 full plate film sheaths that slip into the glass plate holders and hold film. I did have to do extensive modifications to one of the other glass plate holders so it would accept film and fit the camera. I did not have to contend with making a back but don't underestimate the skill and time required to make a functional back.