Floor sensor, instead of door contact


Instead of a door contact I was wanting to put a sensor under the floor, that would detect if someone was standing there. Anyone know of a good sensor that can be used for this?
http://www.sureaction.com/ <- those are the ones I have, and from the research I did, probably the most popular one as well. I got mine from AutomatedOutlet.com. SureAction calls these stress sensors 'Pulsors' in case you are trying to find them. Keep in mind that you also have to buy the 'processor', which can have up to 3 zones (and 1 zone can have up to 4 sensors). SureAction's tech support isn't that great (at least the email support), and the board itself doesn't have any mounting holes or case, which is kind of annoying. Here is a picture of my install (mounted below the SECU16):

Click to enlarge