I think the cross bar to hull connection is similar to the Hobie 18 - so I'll pass on what I did to my 18SX.

Getting the tramp laced tight seems to be important in holding the whole rig rigid. In my case I found the tramp tracks on each hull had worked loose - stainless rivets and aluminum track leading to galvanic corrosion of the aluminum and loose rivets. I only really noticed how loose the tracks had got when I removed the tramp. To correct this we drilled out the old rivets, filled the worn holes in the hull with epoxy/filler and redrilling them, then riveted the tracks back on (the second time we used rivet washers like you should do - the first attempt pulled out after a couple of races ).

Once the tracks were sorted I double grommeted the tramp then laced it with some skinny polyester covered spectra line (I bought a whole roll - it's useful for all sorts of rigging tweaks). This line makes tramp tightening very easy - just don't over do it or you'll rip out the grommits

When I took the cross bars off the hulls to align the rig I noticed the cross bars only make contact with the hulls in a few places (the worn patches). To increase the contact area and hence stiffness I bonded the cross bars to the hulls with 3M 5200 compound on the advice of a wise old local. This gives you enough time to align the rig square as you tighten the hull to cross bar bolts afterwards. A week later I tightened the bolts again and since then the rig seems much stiffer. That combined with some steering tweaks has really improved the feel of the boat.

Chris