I make my shipping boxes by cutting pieces of Super TUFF-R Insulation from home depot, it's got a foam core with reflective foil on one side of it. The sides, bottom and lids are snug, but not airtight. One side is blue and does not reflect heat, so I face the blue inwards during nice shipping temps. I reverse it and have the reflective foil facing on the inside during cooler temperatures to help keep the heat in. I use Kordon breather bags for all my fish, ship mostly priority, using 72-hour heat packs wrapped in a sheet of newspaper and usually make an extra pocket inside the box made out of the styro insulation but only as high as the heat pack, so that it will never touch the bags but air can circulate around the heat pack. I do not ship in very cold temperatures. I don't know anyone that does. I have not had any issues shipping fish this way.
With you using actual snug fitting medical styro boxes, you should punch out a hole in the lid of the box and tape the heat pack over the hole, on the inside of the box, without wrapping it. The air from the hole will keep the heat pack going. That's the way the pros do it! With 7"x7"x6" USPS priority boxes, it's just not practical, there's not all that much room leftover after you get a bag of fish in there.
With you using actual snug fitting medical styro boxes, you should punch out a hole in the lid of the box and tape the heat pack over the hole, on the inside of the box, without wrapping it. The air from the hole will keep the heat pack going. That's the way the pros do it! With 7"x7"x6" USPS priority boxes, it's just not practical, there's not all that much room leftover after you get a bag of fish in there.
