Does your hybrid have the pre-attached tent ends? The ones I've seen don't have such gaps.
Popup Gizmos make a thermal 3-sided bunk end insulation insert.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package