If you want to be sure it won't go anywhere, get some wider then the bracket you have flat plat as already mentioned and make it go from outside bar to outside bar. Then either get the ends folded down at 90 degrees so it wraps around the edge of the bars or get it welded with a 90 degree bend on each end (same as bending the plate idea). You can then bolt the bent down end bits through the sides of the bars (probably 2 bolts either end) and also screw the centre sections to tops of bars it crosses over in the middle. Or if you really want it not moving.... get some 90 degree bits welded onto the plate at the sides of each centre bar and bolt through the sides of them also (you may need to mark the holes and remove plate to drill though). Then attach the thing that holds the forks to that plate as mentioned previously also.