Here is what I did to mine.
First of all I ditched the long through bolts and put individual bolts and nuts holding the top beam. The through bolts just deformed the C-channels and wouldn't provide adequate clamping. Also note the clip holding the jack handle, convenient and out of the way.
I added a knob to the jack valve, as you did.
I added cross bracing to the bottom of the press, That and the new bolts at the top eliminate any racking of the frame. I give up some of the lower press points, but I've never had a need to use the press for anything that requires that much room. I also secured the press to the floor.
I used thick, stiff adhesive backed felt pads to take up the slop between the movable part of the press and the side frames. Hard to see in this picture, but the stabilize the moving piece and don't interfere with it's movement.