LnS,
Both the Spits I've worked on had nested square-section main spars, built up from the smallest section, heat expanding the next size up, then sliding on to the first.
As the outer cools, it grips the inner very effectively!
ISTR that there is a solid centre near the root, and that the bottom edges of the spar are progressively cut away towards the tip.
I *think* that the Hurri's spars are built up bit I can't swear to it off the top of my head!