Had a similar situation with the daughters truck. Solition was a Fosgate 10" sub in a custom box. The back of the cab is cut out and the opening reinforced with a little angle iron. The sub box was slid thru the opening almost all the way back to the front of the bed. This setup solved 2 problems: Got rid of the ugly bump on the back cab wall. And allowed me to build a 1 cu.ft. box. For your install, you might consider a shallow mount sub and build a .75 cu.ft. box.