Get two bolts with Flat heads on them and screw them partially in the holes. Put silicone gasket on the heads, hold it in the air and shove it all the way against the corner where it sits and let it slide down till it touches the trunk floor. Raise it straight back up and you should have two marks. Take a uni-bit and drill them oversize so you have room to play with underneath. Then get some big flat washers and re-use bolts and have someone on top to move box in place while you bolt it in. put silicone on the backs of washer to seal out moisture and your done!
