Erm, when i put my hard top on I used lengths of stud iron through the mounting tube things, and also my side panels have what looks like a retro fitted bracket thing half way down their length, which is very effective at closing this gap. You may find that tightening up the bolts at either end stretches the tub or the sides outta shape, so giving this gap. You could then try fitting a bracket which bolts to the capping to solve this, i mean it worked on mine, fitted by the old owner of the side panels.