I just love to make guards of posterboard-type material I get from packaging, heaven forbid I should pay for the stuff. Fold goes along the spine, then I staple along the contour of the edge for a relatively snug fit. (though they will likely slip if just thrown loose in a case). Put some self-stick foam at the mouth if you want a fancy snug fit.
I haven't done it myself but I'm sure you can do the same with 1/8" chloroplast (corrugated plastic used a lot in sign making). This would I think give you the as nice, tough, long lasting and cheap a knife guard you can ask for. Plug up the cells with RTV if they offend you. The stiffer material also probably provides more grab onto your knife, aside from addition of any foam. Just collapse the cell where you want the fold to be using something like the handle of a butterknife and it will easily fold a neat 180 deg. Just make some paper templates so you don't waste good material.
For knife totes I you can do a quick search here for knife rolls/bags/cases/transport.