Heat pad question

[Shmoges]

I am going to be shipping one of my snakes soon and I would like a recommendation for a good non toxic heat pad that I can stick to the inside lid of the box.

I have a card board box that has styro foam insulation and is vented (saved box from snakes that were sent to me). It has more than enough room inside for him to move around in a bag and not come into direct contact with the heat pad unless someone puts the box upside down..(it has arrows all over it for this side up).

The temps where I am are in the 60's during the day and 30's at night so I am thinking a 8-10 hour heat pad for overnight should be good?