You need to weld an angle iron or a hoock-like something to catch the bucket's edge in the front of the rack to prevent the bucket sliding forward. The strap at the rear/top of the bucket won't suffice : in a hard stop or an accident, or just vibs, the bucket will slide forward while rotating backward (nose lifting up) and will fall off, even still tied.
My second point is the grapple. Its mounting plate is a serious obstacle , either in or out of the trailer (actually both). Maybe you can bust your head on the bucket's rack, but I'm sure you can do the same from the outside with the mounting plate or wreck a shoulder/arm from the inside. Too dangerous.