It's the cooling elements size.

The box size is: 300 x 300 x 250 (outside)

After the insulation the box size (inside): 270 x 270 x 2000

L x W x H

OK .... let's find the AREA through which the heat will be lost .

two faces are .3x.3 = 0.18 ...four faces are .3 xx.25= 0.3

total area which transmits heat is 0.48 sq meters

from post 2 we know heat loss through a one meter cube is 0.95W @ 25 deg difference .... but our surface area is less than 1 sq meter ... 0.48 , so 0.48x 0.95 =

**0.456 W** this is the rate of heat loss if the walls were 1 meter thick ... but they're only 15mm thick !

so the heat loss will be greater by 1000 divided by 15 = 66

**so the rate of heat loss is 66x 0.45 = 28.5Watts **
Latent heat of fusion of ice is 334 joules per gram, that means that one gram of ice can keep your box cold for 334 / 28.5 = 12 seconds

1 kg of ice will take about 3 hrs to melt , keeping the inside of the box at 0*c for 3Hrs

Check you have measured the wall thickness correctly , the figures you gave give the top and bottom as 25 thick ... if this is correct 1kg ice will last about 4 Hrs .. if you double the wall thickness you double the time the ice will last.

24 hrs will require 6 kg of ice (plastic water bottles will work OK ) ... rather a lot of ice perhaps think about thicker walls , or covering the bozx in a duvet or blankets.

Cooling elements ??? I guess you mean those blue plastic things containing jell , will not be the same as ice... I don't know if they will be better than ice or not??