What they are afraid of? Maybe it is matter of area needed to press negative to be flat. I don't think it would be enough to press negative only on two sides (lenhgt) to be flat, negative need to be pressed on width sides too, and non picture area of negative width is maybe not enough... Who knows?

If nothing helps, use glass carrier, but be ready to fight dust... Dust is reason why I agreed to 23x35 instead of 24x36 negative carrier size...