Here's an idea that I liked so much I used on my truck. Not wanting those nasty tailgate chains rattling around messing up the paint, this '66 SWB Chevy uses a set of the stainless seatbelt latches (from mid '80s GM cars and trucks). The seatbelt tab is welded to edge of the tailgate and latch is bolted to the bed post. These cables are custom made from plastic coated wire rope, but I see no reason not to use later model pickup cables. They ready made and easily replaceable and maybe the attaching hardware can be used instead of having to make the brackets used on this set-up. The cool thing about this idea is that it costs almost nothing.
As you can see by the pictures below, I welded the seatbelt tabs onto the tailgate along with a couple of fabricated tabs to attach the cables. The seatbelt latches are from an '83 Suburban and are the stainless steel variety and are bolted to the bed with stainless steel hardware. I used Ford cables because Chevy and Dodge pickup cables are too short. Suburban cables are long and would work too. The bolts to hold the cables are for a Dodge pickup because the Chevy dealer was out of them. It closes nice and solid but if you have short arms you might be in trouble because you have to press both seatbelt buttons at once to open the gate. On the bright side, it keeps small people from stealing stuff out of your bed!!
Troy Baker's method of holding the tailgate shut without chains is another money-saving idea using Overton's parts. This latch is a triple-plated brass assembly used as a hold down for boat hatches.