I did some testing with an additional 40mm fan blowing air across the system board. This fan brought the temperature, even under continuous maximum load and fully insulated, down to between 30 - 40°C. This was deemed more than acceptable.
I fashioned an "L" bracket out of a length of metal bar with 3mm threaded holes along its length. The fan was attached to this bracket using a pair of 3mm screws.
The fan and bracket were used to mark the mounting holes for the bracket. Once drilled, the holes were countersunk so that the screws would sit flush to the box.
Here you can see the fan mounted on its bracket. The fan blows across the heat sink. It might not look much but it proved to be highly effective.
Wires for the power switch, power LED and HDD LED were soldered to their respective pins and then the covered with heat-shrink sleeving.