fidoplank10

I got this T100 cheap, with check engine light on, and cracked bed. Prior owner had a heavy lumber rack with 5' overhang, which would oscillate at frwy speeds until the front corners of the bed started to peel back like a can of sardines. Took the bed off and had it professionally welded. All good until I mounted a pop up camper with 3' overhang. Same oscillation again (I guess the frame is flexing at the joint between cab and bed).
Is there a product to insert in the space between the cab roof and the camper overhang that might quell the flex. I've seen some big ass campers with shocks for this reason, but I don't wanna drill holes in the cab. Maybe some kind of inflatable ring (kiddie pool), or rigid foam?