I 'll post a picture of the one I made for the same purpose when I get to the computer later.
I got a 3' sqft x 3" foam and cut out the center to fit the window and cargo light. Fit some wedging to take up the slope and used spray adhesive to hold together. Then reinforced the unit with black gorilla tape. I slip a cat box liner over the top for rain protection.
I made it from 3" thick foam. The bottom rests on top of the front of the truck bed wall. You can put a couple pieces of duct tape from foam tunnel to bed to hold in place. I usually don't need it. On my truck TC the gap widens between the two as you go up to the top of the cab. I measured, fitted and cut the wedges to be about a half inch thicker then the mated truck and TC. The foam is glued together with spray adhesive and limited overspray. We then used Gorilla brand duct tape to reinforce the unit. I guess it's about 5 years old now and still works good as new. I made the wedges about 1 1/2" thick so they would compress between the cab and TC and conform to the curves. I place a plastic bag over the top to keep it dry. We have never had a leak problem even after hours of pouring rain at highway speeds. It has had no trouble with flexing while driving and it has left no marks of usage. I have 1 1/2"rubber spacers on the front so when loaded, the TC consistantly has a 1/2" gap
between the top of the front bed wall and the TC. You can accomplish the same laying a 2x4 across the front of your bed to achieve a consistant gap if need. Our cat loves it. It gives him that little extra space to roam and be free. He usually goes up to the bed and climbs under the covers when in transit. It's the only time he gets under covers. Be sure to cut out for your cargo light. Hope this helps.
Side View.

TC Side.

Truck Side.
