Gotta agree with everyone else. That's a terrible spot for a safe. You're better off keeping your vehicle as inconspicuous as possible. That means no gun mfg stickers, no NRA stickers, nothing. Keep it plain and boring. If it looks uninteresting, the thieves will pass it up. Take this for example.
When I was in HS, I had a 1993 Chevrolet 1500 pickup, with a single cab and a short bed. It had the 350 V8 in it and of course it had a decent exhaust. I was dual enrolling at the community college my senior year, and one day somebody tried breaking into it. Looking back, I know why. It was loud, fast, and the dark blue paint was brand-spanking new, waxed and polished to a showroom shine, and all the chrome (factory only, I don't add unnecessary chrome) was polished extremely well. They tried using a slim-jim on the door. They grabbed the wrong rod and just broke my door handle. Let me tell you, that was a pain in the ass to fix.
the truck is now mothballed in storage and I will resurrect her one day, but now I'm driving a 2005 white tahoe. It's loaded to the nines in options, but from the exterior you cannot tell a thing. Except maybe the sunroof
Only GM nerds know what that would mean. But on the exterior, it's got a plain, boring color. No matter how much I wax and glaze it, white just doesn't have the same effect as a dark color. It's still boring. But I've left all kinds of valuables in it, hidden out of view, and never had a problem. It just looks boring. Given that it's the "old body style" now to the kiddies who missed the 80s and 90s, they're going to pass it up.
Point number two. I bet you're tired of reading, but just stick with me for another little bit. You said you're driving a toyota matrix. It's a modern vehicle. Do you know how difficult it is to steal a modern vehicle? Most, if not all of these "new" vehicles have the smart ignition keys with computer chips in them. You can't hotwire those. at least I'm not aware of how you could, if it is possible. Getting into the car would likely mean breaking a window or disabling the lock. In most modern vehicles, this would sound off the horn. They'd only have a few seconds to get into that car.
A floor safe under a seat would be fine. They will get caught if they sit there and fiddle with it.