I start with a sheet  of 8 x 4 foot  best-quality 12 mm thick ply and get it cut into 6 rectangular strips. There is some left over.  Often the timber supplier will have a table saw and will cut for you. I recommend best quality ply because it will have less air-gaps and so be stiffer at the jaw-line. Stiffness makes filing much more predictable. 

Note an axle-bolt at bottom right of image below. That allows the vise to rotate to just the right angle and the right height for you.

Cutting list

Whatever vise length you opt for, cut your ply  to these widths:-

6-cheek   vise		No	width		totals
	inside cheek	2	250	mm	500
	middle cheek	2	229	mm	458
	Outside cheek	2	208	mm	416
	tool tray floor	1	150	mm	150
	Cantillever fillet	1	30	mm	30
					1554

Make a curve

The curve of your vise should match your most commonly-filed saw. Improvise a compass …