Free: Contests & Raffles.
$5 worth of hardware is what i have ALWAYS used to keep my dog in the back of the truck. An eye bolt, Clip and sturdy chunk of rope that is just short enough they can't hang themselves.
Update: he seems to be doing well. Has his front shoulder wrapped up to heal. Hopefully no surgery is required. I took him a smoked beef bone last night, he was happy to see me. I was afraid he would shy away from me but he didn't. I am going to either crate them up or put anchors in my bed. Pretty sure the anchors are the way to go.