On the outside of my floor I also used a plate.I drilled holes in it so the bolts go thru,then added a washer and nut to each bolt and tightened them up.My thinking on this is to help spread the load on the underside of the floor as well as the inside.Maybe a bit of overkill,but it was quick to make and cheap.Plus its a good sturdy point to place a trolley jack.Regards,Micky