Building a solid box just adds more weight than is necessary. The same strengthening effect can be had by cutting large ovals into the plates you add before they are welded in, leaving an inch or so on the top and bottom to add the necessary stiffening. it's a trick I learned long ago when beefing up dirt tracker frames. The box section adds the strength, and the cutouts allow weight to be concentrated where it's needed instead of just making the thing a lead sled. To make the cutouts uniform, use a large hole saw to form the ends of the slot, and join them with a straight cut with a sawzall.
The cutouts also solve the problem of making the channel area inaccessible for tubes, etc.
Jerry
