Don’t rush into my version of double-jaws of plywood. Consider if you want a single- or double-jaw and whether you’ll put it on a table or on legs. If it goes on a table choose a filing height that works for you – or suffer a sore back.
I prefer the rotating double-jaw vise on legs with rounded feet. I like it to rotate because the vise will hold the saw in positions that my body cannot. Also hammer-setting teeth is far easier when the saw is horizontal. Because the vise-height is varied by opening or closing the legs different people can use it with the same comfort. If you ever share a vise and file with others this matters!
Rounded feet mean the vise will sit on uneven ground more easily and be stable over a range of different leg-angles.
Plywood is cheaper and more readily obtained and shipped than knot-free, straight-grained twist-free softwood.
If you know you really need a long vise then cut your strip at the widths here out of the 8 foot side of the ply. You could make it up to 8 feet long. I find 4 feet is plenty wide enough so suggest you just start with a 4 footer. If you really need length you can place two 4-foot vises side by side