waterproof pressure switch

This post is for the electronics engineering side of a project to figure out an inexpensive way to make a waterproof mat that would survive an indoor environment for a few years, that would act as a pressure switch providing a simple "on" signal when a weight of at least 5kg is placed on it, and revert to "off" when the weight is removed. The most weight on the mat would be about 150kg. Other constraints: 1. The idea is to mass-produce these mats: 5,000 units or more. So where I say "inexpensive" above, I mean inexpensive to mass-produce. 2. Weight measurement is not a goal of this project. The switch does not need to trigger at exactly 5 kg. Anywhere between 3 and 8 kg is fine. As stated above, it is just an on/off. 3. Economical use of electricity is required. The switch will be battery-powered. 4. I would like to expect the mat to last for at least five years, 20,000 activations. Some drift in sensitivity is tolerable during this lifespan; for example if it requires 8 kg to activate in the beginning but only 3 kg after years of use, that is fine. Also the contrary; if it requires 3 kg at first but 8 kg after years of use, that is also fine. I have no goal of planned obsolescence: if it lasts 20 years, that's great. 5. The weight might be applied across as little surface area as roughly 18 square centimeters up to roughly 450 square centimeters. There is some correlation between the surface area and mass of the objects we want to detect. If it is helpful to the project I can figure out how much of a correlation there is and come up with a number of pounds per square centimeter, rather than a total number of pounds, that we desire to trigger the switch. 6. Total surface area of the mat should be about 1,000 square centimeters. 25cm by 40cm would be a good size. 7. The electronics would be sealed inside some waterproof covering to avoid both water and physical damage. The design of such a covering is not within the scope of this project, but the electronics design should be such that enclosure is expected. If significant heat is expected to be generated and require dissipation, this should be noted, but hopefully this will not be a concern as low electrical consumption is a design requirement. 8. Anticipated height with enclosure should be less than 1cm, ideally; up to 1.5cm is OK. 9. The "on" signal should be sufficient to control a MOSFET relay, preferably 5V. One idea for the pressure sensor is to use a material such as Velostat or Linqstat that reduces resistance under pressure. However, I need you to design the circuit for it. The circuit should minimize battery consumption while also obtaining a strong enough signal from the mat to minimize false positives due to stray voltage from passing mobile phones, WiFi and mobile antennas, or a standard U.S. 120V 60Hz electrical cord laid across the mat passing 10 amps or less. If you were to design around this material, I would also need you to locate the specifications of the Velostat / Linqstat material and confirm that it is expected to survive the above-described use case.