[rhagfo]

Well we have floor ducts for our furnace, all off a central floor duct. This places them close to the center of the floor and in the traffic path. We have moved from the cheap pressed steel registers, to a heaver gauge stamped steel register, but they all bent, down in the middle and up on the edges.
Well we are pretty sure we have found the fix, DW found these at Lowes, Cast Iron floor registers! Not terribly expensive at about $27 each.
They do set a bit higher then the others, but for the strength difference a great trade off. I could cut the vinyl plank flooring and set them down the thickness of the flooring.