Cargo Box Overhang?

I scored some cheap Thule crossbars and feet on Craigslist and plan to mount them on my topper with Thule's artificial rain gutter. Would like to get a box to install on top for a little extra room to sleep on my camping platform in the back.

My shell pops up a bit (which is great for the additional headroom). Front to back, the top of the shell with the bump in height measures about 40". This is significantly shorter than the length of a lot of cargo boxes (which mostly run in the 70" - 90" range). That could potentially leave me with 30" +/- of cargo box hanging off the front of the shell with about a 6"-10" gap between the bottom of the cargo box and the cab of the truck (depending on how high I mount the cross rails off the shell top).

Question is, is the cargo box going to turn into an airplane wing with that much overhang or what? I'm already guessing that my MPG will take a hit, but seems like it could get drastic with that much.

Adding racks and a basket and deflector or whatever onto the cab to meet the bottom height of a box is financially out of the question right now.

Anyone else running/ran a box on a similar topper? what were your results?