Sand paper and flat table here as well for width and thickness but have used a Dremel cutting disc to shorten it to approximate length then file and paper to finish. Blue painters tape (I use 2 layers and just get into the first layer then stop) in the bottom groove if need to “thin” the bubble strip down some to fit the clamp. There are variations on how to hold it in, I personally slightly bevel my sides and it slides firmly into the clamp without wedging itself stuck or needing just the right width to be pressure held in place with the clamp “jaws” (which is hard to do when it properly fits the graflex body).
As for the BP, pressure fit onto a cone of some sort, tape again would work well to hold. Don’t glue it unless don’t plan to ever take part without difficulty.