A hollow box structure is indeed the most common form for a backhoe
boom. JCB made a few with a lattice construction in the 1950s, but these
are the only exception I've seen. I know you're probably building it for
your own use, but make it strong as backhoes lead a hard life! Have you
seen an operator lift the rear of the tractor using the backhoe, then
shift it a foot or two to the side? With a strong backhoe it's a
tempting thing to do, and of course it reverses the loading on the boom.
If a boom or dipper fails, it will usually crack through the central
pivot. The pivots are the places to add extra plates for reinforcement.
The best advice I can give you is to look at some backhoes closely and
see how they're built.

Best wishes,

Chris