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There would be nothing stopping you from clamping such a jig in a vice and cutting vertically - the advantage of having it horizontal is that you can apply pressure straight down onto the blade of...

Sure thing!

It's nothing special, just a flat piece of plywood and two guides that are fixed with double sided sticky tape. Clamp the blank down and use the guides to cut - not hugely fast but...

I managed it by making a jig and cutting the blank down using a fine Japanese pullsaw - it was fiddly but it worked in the end.