Home Made Cool Box Mash Tun - Extra Insulation

I am getting ready to brew again, just waiting on some more grains arriving and so in the meantime I've been trying to address some of the problems I discovered during my first all grain brewday.

Firstly, my homemade, cool box mash tun lost around 6 degrees during the 60 minute mash and the mash temp was only just at the optimum temp for mashing (65c ish). The recipes in Graham Wheelers superb Brew Your Own British Real Ale (Camra) call for a mash time of 90 minutes so the temp loss will be an even bigger issue this time around.

Luckily I found a few slabs of 30mm polyurethane foam in the barn and set about building a tight fitting box around my cool box Mash Tun as well as a tight fitting lid with an additional thickness of foam inside.

The box was relatively easy to construct and is held together with some long screws. I made the lid so that it sites over the top of the box which also means that around 30mm of the sides around the top have a double thickness of foam too.

I haven't tested it yet as I've only just come in from finishing it but I think this additional insulation around the mash tun will make a great deal of difference, especially the 60mm total thickness in the lid (where most of the heat is reportedly lost).

PU Foam Insulation Box for my Home Made, Cool Box Mash Tun

EDIT: Well bollox! After all that it's only made half a degree C difference over 90 minutes. Started test with water at 65C. After 1 hour it was 61C and after 90 minutes it was 59.5 C - FFFUUUUUUUUUUUU

EDIT TO EDIT: Ignore the above, it seems testing a mash tun with hot water only gives less than favourable results. The grain in there also adds to the heat retention. This was proved in my subsequent brews, brew 2 All Grain HomeBrew Courage Directors and brew 3 All Grain HomeBrew Irish Stout when I lost just 1C  on brewday 2 and there was no measurable drop in temp on brew 3 :)