this might be interesting to read:
Two Teen Boys Cling To Back Of Tractor Trailer On Freeway - https://www.autoblog.com/2013/08/15/two-teen-boys-cling-to-back-of-tractor-trailer-on-freeway/#close-modal
and maybe (just a suggestion) with a bit of cardboard and some spay paint make a cover for
the hole in the top there that you could "seal" yourself in with to avoid detection. of course, this would be extremely dangerous, you would be trapped, and in big trouble if detected, witch will probably happen, because:
1: the person driving will know the truck well, and notice the change
2: you are laying on a thin sheet metal roof. these tend to make loud pops when weight is applied.
3: you will probably be seen or heard climbing up into the hole, as most trucks with the hole there don't have ladders, most likely to prevent people fro stowing away there
4: you peak out to see what is going on, moving the cardboard, and alerting nearby motorists.
and 5: your screams will be easy to hear when you get frostbite from the cold metal, hit your head, get heatstroke or just get frustrated because you are stuck in traffic with nowhere to go.