Make a Harbor Freight Bed Extender taller?

I was wondering if anyone has found or fabricated a way to extend the Harbor Freight bed extender to match the height of KB voodoo hi rise bars ... I'd like to carry 2 kayaks on the bars and use the bed for other gear, but the Harbor Freight extender stops at the level of the bed.

 

I'm assuming you have this one? You should be able to get a piece of square tubing the right size(s) to extend it up higher. Not sure how much play there would be in it, though.

 

I would imagine that the Harbor Freight product is made with lighter weight material. I think you would be much better by building your own out of Schedule 80 material.