Cap and rack clamps.

Google is your friend, look at "Truck cap clamps"

Like these, $5.25 each AC1010 by API?

Oh, and, here is the full API brand listing by vehicle make and model

None of these fit a ladder rack. They don't open wide enough. All I can find is these expensive beauties at a few different sites. $80 for four.

I'm temped to get me some 4" c clamps at home depot. I'm trying not to bend up my bed rails.