If you were to install the switch and button anywhere that's not obvious, you should be OK. You could also run a kill switch in-line with the switch that gives you juice and hide just that. Think about it: only a few people have a push button starter so I don't think a thief would be inclined to look for one. They need to get in and out in a short amount of time. If it's not obvious, they're going to go for your ignition and see that it's frozen (I assumer yours is frozen) and say WTF??!!
Buy one of Prez' kits - as long as someone you know is somewhat mechanically inclined and has a few tools, you'll be all set. He's got good directions and his kitss cheap. By the time I went out and bought some the tools and equipment I needed to make all the parts, I probably ended up spending 2x what he's charging, plus time to put it all together - BMW wanted about $600 to fix mine, so for less than $50 and probably a 1/2 hour for installation, you'll have your problem solved.