They're typically called "tool chests" or simply "bed chests". "Low profile" usually refers to the height above the bed rails on a crossover toolbox (aka mounts to bed rails).
There's multiple brands out there...Delta, Kobalt, ProTech, DeeZee, etc...blahblahblah. Alot of it depends on your budget. You could be just fine with a plastic lockable random brand storage box. Best way to mount it is using nutserts or threadserts as they're often called. The flanged ones work best on the bed floor.
Here's what I first thought you were after:
Here's what I run now. The biggest floor mounted chest I could find that would fit. Can't even remember the brand but was purchased through either Northern Tool or Tractor Supply. It actually sticks up past the bed rails almost an inch....so it isn't doable with a tonneau cover.