This is based on a chinese product I found, but with some improvements (no drip tray required, no ice machine required, and higher voltage and larger fan for more air flow).
Basic idea, air is sucked in through the back, through the box which contains frozen ice packs, and blown out by a fan.
Designed with hole for 14cm fan powered by 12v source.
I designed and printed before the ice packs arrived, so made space for 3 lots of 17.5 x 9.5 x 3.5cm ice packs (https://www.amazon.co.uk/gp/product/B00CMOZYMG/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1), but once they arrived and I froze them, found all 6 fit in nicely anyway.
Tried a few different fans, but the one that works best is a Static Pressure type. These are generally quite expensive, but found them quite cheap on ebay sold as New (other) for £6. Much higher RPM and forces air through the ice packs with much greater force.
Works better than I thought, but you'll need spare ice packs to swap in every hour or so.
Recorded 10 degree Celcius temperature drop from 20cms in 10 minutes.
Found air cools better with increased airflow if fan blows into the box, rather than blows out (something about compressed air cools when it is released, but won't get into that). Not a big deal, just spin the box round.
Also made the 'Grid', though it's more a Guide for the ice packs so they don't fall on top of each other, and leave good spacing between each. Just stick it in with glue/tape.
Modified with thinner walls, simpler lid, built in fan grill, and removed ice block guides. Still suggest printing two of the guides to fit as you wish
I swapped in a 0.6mm nozzle because the 0.3mm nozzle was saying 24 hours to print. Forgot to adjust perimeters after doing this, so used a lot more filament and time than planned (11 hours). With correct settings, should be closer to 6 hours.
With 0.6mm nozzle, 1 perimeter, 1 bottom, 2 top, 0.4 resolution should be plenty strong.
Double up all settings for 0.3mm nozzle.
Preferably PETG if you have it