I'm a big fan of laserboost, I've bought three plates from them and they were all flawless.
http://www.keyboard-layout-editor.com is pretty useful, if you can get your layout as right as you can figure it in that, then export it to cad, then check how it measures up with a set of digital calipers.
That's how I made mine, suppress the urge to cut corners, just be patient and measure again and again. The normal space between keys is 100% standardised, you've just got to work out how many units wide your caps are, and then it's just a question of locating your clusters of keys.
Free CAD software is pretty rubbish, but I've always managed to do what I need to with QCAD, it's honestly pretty painful to use, but it's entirely doable.