Can't see anything except for Duct tape ....
Talk about all wrapped up!
I'd be persisting with the cold finger project. A cool box has very limited effect on sensor temp as you have probably noted. You are using a lot of amps and still not getting stable temps where it matters.
My 450D runs about 1.2 amps and maintains 5* ( ice prevention temp ) within 10 minutes. If I wrapped it up it can achieve -20* within 15 -20 minutes.
A '99' error is a bit of a show stopper though. Is Glen able to assist ? He seems to be the Canon guru for hacking. Normally it is just a connection problem. I had it now and then on the 450D but just had to take a lot of care with the connectors when reassembling.