>>272993224
If you're decent with a needle and thread you could make some airflow guiding tunnels in the sides of a backpack.

>Cut holes in sides
>Sew fine mesh over holes
>Sew fabric tube from inside face of hole, along backpack inner liner and ending at the fan intake and/or exhaust. 

If the laptop doesn't have an intake and only an exhaust then you'll need to provide some positive air pressure by getting a little USB powered fan and having that sewn into the front/side of the bag, blowing air inwards. 

It's a clumsy and dumb solution but it isn't actually an insurmountable issue.