And success !
Small piece of rubber tubing slotted into the radiator fins and directed at the front window through the slot in the 2" extender body.
Ambient temp at start was 22.2*. I got down to -1.8 at 47 mins and 12 volts. ( after 20 mins at 5 volts to see how it went ). Held temperature for an hour and 40 and no icing up or dew internally or on the front window and a clean good image of the houses on the ridge 5 km away.
Interestingly when I finally powered off the temp was back up to 11* within 4 minutes which just shows just how much heat transfer is going on. I'll improve on some of the insulation layers as well to minimise that.
I just need the clear filter window, some fresh dry silica bags and we're ready to have a go at some DSO imaging. If I can't get a clear glass window I'll grab a UV\IR filter instead. If I can get a 24* delta on a 10* night I'll be more than pleased but I reckon there is a bit more in it yet.
Here's a picture of it up on the scope.