I used the bubble wrap insulation from Lowe's it was about $50 and covered everything I needed it to. Not a bad option as long as you cover the gaps between the topper and truck bed.
Do you have a bed liner in your truck?
I threw away the bed liner when I got the shell, it came with the truck. I put insulation then plywood on the floor to level it out. I have a work package edition, it doesn't have a carpet interior in the cab either. It has rubber floors but I still have Weathertechs.