I have your answer!

I was using a hacksaw and elbow grease for my first couple kydex projects. It worked well enough although a pain. But, I knew it wasn't the easiest way.

So, after some thought, I decided to mark my Kydex with a pencil and then put it in the oven. I heat my Kydex in the oven to 190C/374F. That is just under the burning temperature of Kydex according to Kydex's official specs. They say the burning temp is 204C/399F. Then I take it out of the oven and use a utility knife to cut along the lines I drew.

Hot, soft Kydex cuts like butter even with a dull knife.

This also doesn't leave the same mess a hand saw or band saw does. It leaves absolutely no saw dust.

Then just put it back in the oven for a few minutes to get it back up to optimum temperature so you will have the maximum amount of "work time" before it cools too much. This is also why I bring it so close to its burning temp. I don't like to rush things.