I carry a 14 and a 15 in the bed, nose first. Strap the nose down so it can't bounce up, and tie off from each corner of the truck bed back to the handle on the rear to keep it from sliding backwards or back&forth. Works pretty well and only costs a few feet of rope & strap. Also nice to only lift it to waist high, but puts the kayak in more danger of running into something (or being run into).
They make bed extenders that plug into a hitch receiver. Like a 4' receiver hitch with a 'T' that comes up from the hitch and adds another "shelf" a few feet behind the bed. The kayak specialty brands cost about $150, but Harbor Freight sells one as a ladder/lumber hauler for $40 (sometimes less).
Next option would be some kind of "goal post", either out of the hitch or up from the bed rails, to provide a rear support, with foam pads to let the front of the kayak sit on top of the cab. I haven't priced these out, but I suspect if you looked for truck/contractor versions you'd find something that works at a better price than looking for one of the name-brand kayak/canoe varieties (yakima, thule).