If you're not opposed to welding, you could just get two scrap pieces of angle iron (not really big, like a 3 inch long piece on each side, drill a hole in the iron to mount the angle iron to the box, tack (or weld the whole piece)the iron in two places to the trunk floor then screw the iron to the box. That'll hold it for sure, not going anywhere if your tacks are good. You could also make a bolt to mount the angle iron to the floor, but that's drilling for a bolt hole, etc. You can always grind out a tack. That's the cheap way to do it. Paint the iron to prevent rust, corrosion. If you want it REALLY stable you could do that on both sides of the box.