Here's a different approach. Just back from Cornwall (2 weeks of hot Cornish sun!!) and this worked a treat.
Place a towel soaked in water over and round the toolbox and leave in shade/breeze. That way the whole box is protected and cooled from the outside. The ice blocks then only have to cope with cooling the interior items. Frozen blocks stay frozen for twice as long. You can save money on that. To keep the towel wet over a whole day, I placed the cold box + towel in a storage lid with water - the water then soaks upwards.
This also works if you haven't got freezer blocks - i.e. wild camping.
I should have taken a photo!
Another vote for the Coleman Extreme and like others we pre-cool with old squash bottles. 2 frozen in the box, 2 in the freezer and swap everyday for about 3 days prior to the trip. Stuff then stays cool for days. I've been given another tip which is to put in one of those foil blankets you can get in pound shops to act as extra insulation. I've not yet tried this.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.